Stephen,

A piston coming apart like that "CAN" be caused by an intake air leak on that cylinder.

if your seizing top ends over and over again, then make sure you do a proper leak down test on that cylinder giving you trouble. Make sure the head and base aren't leaking. Also check the cases, as well as the Reed cage seal, and the intake boots. Even a small leak will kill power, and lean that cylinder out potentially causing another top end to go bad.

Your engine should be able to hold 6-7 PSI for a good 3 minutes.

Also, if your missing "parts" best to pull the engine out of the frame, and fluch the cases. You can do that with the crank still in place. Flush with cleaner, and then blow everything out with air. Of course the best way to do it is to fully tear it down, but who really wants to do that.

Thanks,
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Charles Gallant