You swapped your old flywheel onto the replacement engine? Very possible the key slipped out of the keyway and the ignition timing is off. The method for placing the key into the keyway is often done improperly. The correct method is to set the flywheel in place on the crankshaft taper and THEN...you gently work the key into the keyway, lightly tapping it down to seat it. Some folks try to put the key into the keyway first and then slip the flywheel down over it. Not good because if the key slips down, it may fall out or worse, the flywheel can be tightened down off center. Go back and check your flywheel key. Even if the flywheel was already on the engine, the popping you hear is a good indication of "out of time spark".

Well, you can use the SEARCH feature of this forum to research the issue. You will find that at the bottom of the opening page. Yes, you might have a blown head gasket, but that is not why your crankcase oil is full of gasoline. The gas in the oil is because your tractor has a gravity fed fuel tank and no manual fuel cut off valve in the fuel hose (research that too). The oil blowing out the exhaust might be caused by the gasoline in the oil boiling off and flowing out through the crankcase breather and right into the cylinder to be blown out partially burned. The stream of gasoline vapors trying to exit the crankcase as the gas boils carries a large volume of lube oil out with it. First thing you gotta do is install an inline fuel shut off valve and turn off the fuel at all times except when you are actually using the tractor. Second thing you gotta do is drain the contaminated oil out and replace it with fresh oil. After doing those things (in that order) you can run the engine to see if it is still blowing oil out the exhaust. You may have to run it about 15 minutes or so to burn out/blow out all the oil from the previous episode. If the oil blowing clears up/stops, you have dodged a bullet. If the oil blowing continues, you may have a blown head gasket. Those so called "pay for answers" sites are about as worthless/useless as those "psychic reader" sites.

***"On TDC the exhaust valve spring is at 1-1/8 in. out and the intake is at 1-3/8 in. out."*** I need to emphasize that "TDC" and "TDC on the COMPRESSION STROKE".......are not the same thing. So, having made that statement, let me ask this: When you are measuring the length/height of the valve springs...do you have clearance, or lash, between the rocker arm and valve stem of BOTH valves/rocker arms???. If yes, then you have most likely detected a displaced valve seat. If, however, you have no detectable clearance or lash at the valve stem/rocker arm of the "shorter" valve spring....the engine is positioned at TDC on the EXHAUST STROKE, and the camshaft is holding the exhaust valve OPEN, thus the "apparent shortening" of the exhaust valve spring. So, let's just double check that before we declare a displaced valve seat for sure. ***"Is this motor worth the money with about 1500 hours of usage?"*** That is something that only you can really decide based on how much you want to spend to rehab this engine. You can base your decision on "cost of repair $" versus "cost of new mower $" if you like, and consider that as an "objective decision" based solely on money spent at this time. But, you will still have an older machine that will have other things to fail/wear out down the road, and sometimes......you can second guess yourself to death in these situations. The bottom line decision you have to make is not so much about the worth of repairing THIS engine. It is about how much time/money are you willing to spend maintaining an older machine/engine? AND, whether you are personally capable of doing the work (or the bulk of it) yourself. As to a personal preferrence or recommendation of a particular brand of replacement mower.......I don't go there! But, there are many, many, many threads in this forum where the yays and nays of the different offerings of the market have been argued/debated/dissed etcetera. It almost boils down to "What color do you like?"

for most people its valve adjust but for a select few its the engine design itself. the piston gets lubed by the engine splashing oil. if you dont change the oil yearly and are not using the correct weight oil, the top side of the piston and occasionally the head also will create tollerances so tight even the compression release wont be enough for the starter to push thru. usually i sugesst pulling the engine apart and cleaning the piston, valves, rings, and valve head. get then super clean abd put it back together. keep up on the maintence and it should run fine. also check the seloniod, they do go bad and it takes all that CCA to get these engines spinning