Tecumseh/Murry Lawn Mower Engine Will Not Stay Running?

I have a problem with a Tecumseh 3.5 HP lawn mower motor. I would like to know the reasoning for replacing the flywheel key. My assumption is that the key shears and the flywheel and crankshaft no longer are "connected" (ie: they each spin freely of each other). I had an incident where I hit a large rock that killed the engine but was able to restart it.

Now, a week or so later, the problem is that my engine will no longer stay running. If I press the gas primer bulb the engine starts but runs for just a few seconds. This tells me that at the very least I have good spark (ignition). I went online to verify my carburetor linkage since I had taken it off. Besides that I made adjustments to the governor but I have not had any luck keeping this thing running.

Maybe the two are not related?

The engine has run like a top for the last 23 years (yes 23) with no real problems until just now.

Thank you.