A few more things that come to mind:
If the spark plug is flooded, then electricity can travel across the insulator of the spark plug on liquid fuel and not make any spark, even if it sparked fine outside the engine.
What color was the spark? White or blue is good, orange may not fire under compression.
If the flywheel nut loosened and the key sheared, the timing could be off. Just a few degrees can throw a monkey wrench in the works, even if everything else looks okay. When testing spark outside the engine, the spark will even look regular and steady rhythm, because the starter drives the flywheel.
Agree w/ compression test. If you don't have one, then judge how hard it is to pull the cord vs. how it used to be. If electric, you can listen to it crank, if it's cranking much faster and the starter motor isn't loading as much, then you've lost compression. I've seen a hardened steel valve seat come out of a B&S engine block. Ran fine, shut off when the seat came out of the block, never restarted.
Compression testers are ~$20 and a good diagnostic tool. If you buy carefully you'll get a leakdown tester too. The hose that runs from the spark plug hole to the gauge will be connected with a standard shop air quick-connect. Screw the hose into a sparkplug hole and plug into your air supply. This can also keep valves closed when still in the engine if you're taking the springs off to put new seals on the valve stems.
Good luck, and please let us know what turns out to be wrong with the engine.
:buddies: