crankcase oil level OK?

might try removing the spark plug and then turn the engine over by hand to verify crankshaft and/or other internals not locked up.

then post back with details if still trouble.

That should have a-

Briggs 31P777-0299-E1

for those who are interested.

Could just have rocker issues...

Most engines that go "BANG" are not going to run again. I have had a bunch that went "Bang" and got sent to the scrap yard!
That said---check the oil in it first. Then see if you can turn it by hand, by grasping the pulley underneath the tractor, and see if you can turn it. (I'm assuming it is on a lawn tractor.) If the electric starter won't turn it-- uh-oh!
Most folks make the mistake of removing the dip-stick, looking at it, it has lots of oil showing, so they insert the stick and try to start the engine, with disastrous results. One should remove the stick and wipe off all the oil that is on it, then reinsert it, and remove it again, to get the true reading! Oil splashed on the stick from the last time you ran it, will stay on the stick, giving a false sense of security, and maybe giving a loud BANG!

IMO Probably valve seat which would result in no compression on that side??? but just guessing. I would try to do compression check along with removing the valve covers and watching the rockers move as you crank it over. Should find something. suspect one exhaust valve rocker not moving as much as the others. IF so you may have to pull the head see if the valve seat has come loose, if so reseat and stake, maybe it will stay back in. I would also make sure the heads cooling fins are clean and not plugged up otherwise it may over head and knock the valve seat back out?????