OT - lawn mower engine prioblem

I have a 12 hp B&S engine on a 52" mower. The other day I had been mowing for about 1/2 an hour when

1. there was a sharp "pop" (not as loud as a backfire).
2. The mower slowed (some loss of power). Over the next 5 - 10 minutes
3. there was a couple of "backfires" (not through carb) and
4. it continued to lose power.

I stopped and let it sit a day or so. When I restarted it

1. it pulled more easily than normal (?) (recoil start). It ran pretty much normally for 10 - 15 minutes, although somewhat slower (?). Then I was finished.

What happened?

Valve? Head gasket? (I don't hear any puffing when I turn it over.)

??

Thanks, Bob