I'm guessing it's power valve or power valve linkage.
I've heard of the power valve actuating arm vibrating on the crank case.
Could also be something is loose on the power valve mechanism and the power valve is just flapping around.
First thing I would do is remove the right side power valve cover and run the engine, see what you can see.
Could also be a snapped piston ring but I doubt it as you say you have good compression. How many hours do you have on your piston kit?
It would be worth removing the exhaust and looking up the exhaust port with a torch, look at the piston/rings and also see if the power valve looks OK.
Fingers crossed it won't be anything too serious or expensive.
Best of luck and keep us updated. Dave.