Start it up, hold a card against the end of exhaust pipe, the pressure should hold the card away, however if the card is pulled back against the exhaust now and then, you have a burnt valve, most likely exhaust valve.
If there was a hole in the piston or top of piston broken off, there would be huge blowby, A broken plug or a broken glo plug can cause bent valves
Cheers, the ferret.