You might check the tightness of the sump bolts first.

Here's the scoop: If the engine is in a tractor, you'll have to remove the 2 step pulley under the frame. then disconnect the fuel line, the throttle controls on the engine, then the 4 hold down bolts. Drain out the engine oil. Replace the plug. Remove any sheet metal that might get in the way of lifting out the engine. Have a place where you can set the engine upside down on it.
Remove the bolts that you will see in the bottom section of the engine. Carefully pry the sump up and off the crankshaft, and lay it aside. DO NOT TURN THE CRANKSHAFT!!! After you get the sump off, carefully remove any pieces of the old gasket.
Used sparingly, some heavy wheel bearing grease, spread a light film of it where the 2 pieces mate up. Then lay the new gasket in place, pushing it down into the grease. (This operation is done on the block area, not the sump.
At this time, lay the sump in your lap, and using an awl, or some kind of pointed tool, punch a hole in the old crankshaft seal, and using the tool and a skinny screw-driver, pry out the old seal. Clean the area where it sits. Then make sure you install the new one just like the old one was, in there, using a large enough socket to use as an installer, and tap it into place until it bottoms. (One side will have little rectangular depressions in the rubber/plastic part, and you must get it in the right way. To tell the right way, look for the place where you made the punched hole. Compare it to the new one, and you'll see what i mean.)
After all is ready, slide the sump down over the crankshaft, and on down so the cam shaft goes into its hole, and slide the sump all the way down. This part is a no-see-um, so be careful not to wiggle, turn the crank, or do anything to get it out of time. Tap the sump down sort of tight, install the bolts and tighten them up, then re-install the engine.
And, if this boggles yer mind, better get somebody at a shop to do it. Its not rocket science, but it is sort of tricky. And, if ya goofed some place, all you have to do is do it again!
Rustyj

Bill Kapaun is correct, but you must lift the whole engine to get to some of those bolts, and, if the gasket has deformed, you'd might as well replace it.
RJ

Thank you all. I think I can handle this.

june58, just replaced mine on a 12hp, rigged up a bearing seperater and puller to remove the pulley only to find out some generous assembler at b/s actually coated the shaft with anti seize and it came right off. hope yours went that smoothly.