Crikey, that doesn't sound good. If that's the case I won't be sticking an endoscope in mine any time soon. My piston tops will be like a teenagers face and my valves will be like bananas due to chronic, and repeated, over revving. However, at the moment she seems to be running like a Swiss watch and I'll keep thrashing her until she doesn't! I guess life just isn't fair.
Anyhoo, back to the conversation; if it's only the piston top that kn4ckered, just replace that with a used standard piston if the bores aren't damaged. Is the piston top holed or is there valve damage? Has the piston-top damage been caused by valve interference? Also, if the air filter is blocked I would look at whoever last serviced it as it takes an awful lot of debris to block it to that extent and I would suggest it hasn't been changed for a long, long time. Good luck to you for a swift, and inexpensive, return to robust health.