If it is a leak that shows to be coming only from underneath the chassis, with no visible oil anywhere above the frame.........the culprit is very likely the crankshaft oil seal.
If, however, you can see a trace of oil above the frame that may be running down and eventually dripping off underneath, there are a few other things to check.
Might be the sump cover gasket leaking, and that may be due to loosening of the sump cover bolts.
Also to check are the O-ring at the base of the oil dipstick tube, the governor shaft oil seal, valve cover gasket, head gasket, and the crankcase breather gasket.
Looking under the frame, do you see any oil that appears to have slung off of the engine pulley? If so, the crank seal is probably it.

@mownie...tks for the quick reply; the oil is very visible on top of the frame on the right side of the engine ( opposite side of the engine where the oil filler tube is attached which looks pretty dry at the base/o-ring). I do not see anything dripping down the sides of the engine. The red arrow in the pic shows the area on the frame where the oil is appearing...tks, mac

A couple of more shots...tks, mac

another pic of the front right of the engine where it appears to be the wettest...

A few years back, B&S had a problem with the sump bolts that hold the sump onto the block. The bolts were coming loose. The fix is done by removing the engine, drain oil first, then turn engine upside down, remove sump, clean the mounting surfaces well. Use a small amount of Indian Head shellac, to hold the new gasket in place. Make sure the timing marks on crank-shaft and cam are lined up ok, then lower the sump into place, install bolts and tighten them, and reinstall the engine. Don't forget the oil!!

OK, it sounds like I need to remove the engine to check the sump bolts. Once I remove the sump, which I have not done before, are the internal parts going to remain in place? I saw the ref on checking the timing marks on the crank shaft and cam; are these parts going to move or are we just doing a sanity check? tks, mac

With that much oil on top of the frame I have to say it looks like a sump cover gasket failure to me.
I prefer a newer product known as "High Tack" by Permatex to hold the gasket down.
You ought to go ahead and replace the crankshaft seal too because it is very easy with the sump cover off of the engine.
The crank and cam are not going to fall out of place if you have the crankshaft oriented with flywheel down and PTO end up.

Unfortunately, this project was put on hold since I had pneumonia for the last 6 weeks...argh! Anyway, need to finish it...here's the latest pic. The engine is off and upside down, and the bolts are removed. Is it as simple as lifting the sump straight up? Does it lift over the crankshaft and seal; also, looks like at least one type of dowel pin that maybe hloding the sump bottom to the engine. All thoughts appreciated...tks, mac

here's the pic...

Yes, it should lift off, but there may be some gasket adhesion wanting to be reluctant. Also, if the dowel pins (2 of them) have a good snug fit as designed, they may want to cling.
It is important therefore to try and get the sump to come up evenly on all sides until it clears the dowels (about 3/8" or so).

I've done several of those engines that had that problem. Just time consuming, but not a hard job. Clean any old stuck on gasket material off the mating parts, install a new gasket, and make sure the cam gear is meshed with the crank-shaft. If you turned the crank-shaft while dismantling the sump, better check the timing marks before you put it all back together. A little time spent doing that will save much more time later, if it gets out of time while apart! And, install a new crank-shaft seal while the sump is off.
I have to do one on an older Briggs, with the electric clutch for the mower deck. Not my idee of fun!

The sump is removed; removing the gasket is a challenge, but slowly getting it off. A few more noobe questions:

1. Should you use permatex on both sides of the new gasket; is this just to hold the gasket in place while installing or does this help seal the gasket to prevent leaks?

2. See pic...the 2 big gears have not moved, but I did bump the part circled in red; not sure what this part does, but it is geared and can be easily moved. Does this part need to be in a specific spot?

3. Is there a torque setting for the sump bolts?

tks again, mac

The part encircled is the governor assembly, it does not have a specific timing position.
I personally use a product called High Tack (by Permatex) and I apply it only to the sump cover.
Sump cover bolts torque specification is 220 inch pounds.

Hopefully the last questions:)

1. When I replaced the crankshaft seal, I used a large socket to tap it in evenly. I tapped it until it bottomed out. It appears it is recessed more than the I took out. Was this the proper way to install...tap until it bottoms out?

2. Just curious on the purpose of the hole in the bottom of cylinder that the crankshaft fits into (see pic). Does the hole allow oil to enter the inside the cylinder to lubricate the crankshaft as it spins? If so, then the current placement of the cranshaft seal blocks half of this hole. Is this OK or should I raise up the seal?

tks as always! mac

cylinder hole pic

Well, the seal should only be recessed into its bore about 1/16" below the outer edge of the seal bore.
You drove the seal in a little too deep.
The hole you are referring to is more of a drain hole for oil to fall OUT of the bearing journal than for it to get in.
The idea for having the hole where it is is so that any tendency for oil to be under pressure as it weeps out from under the crankshaft during operation will just exit through the hole instead of powering its way past the seal lip and outside the engine case.
If you can raise the seal up without destroying it it would be a good idea.

I am in the middle of trying to replace the oil sump gasket and the crank seal. The problem is getting the dang pulleys off to remove the engine. I tried an 8" puller but it was too small and am waiting for my new 10" puller to arrive tomorrow. Any tips on removing stuck pulleys? Also, is it ok to clean up the end of the crank shaft that the pulleys are on? There is some rust and I would like to use emery or something just to get it nice and smooth. I was told to put the sump cover back on and then install the new crank seal. Does that sound right? This is my first attempt at this repair and I'm doing it mostly because all the mower repair places are about 2 weeks out from being able to fix my mower. I thought what the heck :)

Good luck on getting the pulleys off. Sometimes it helps to heat the pulley hub area with a propane torch after you get a good pull going with the puller in place.
Yes, you certainly do need to remove rust and any "burrs" you detect on any portion of the crankshaft the seal will have to travel over as you install the new seal onto the crankshaft.
If you are going to remove the sump cover (and replace the sump gasket) there is no need to install the sump cover and then try to drive the seal in place.
If the sump cover is off, put the new seal into the cover before you put the cover back on.
While it IS NOT necessary to remove the sump cover simply to replace the crankshaft seal, Briggs sump gaskets do eventually fail, and if yours is not already leaking, it is good practice to go ahead and replace the sump cover gasket at this time, considering what a bear it can be to get pulleys off.