I had a problem a few years ago with what I though was a stuck piston ring, and I was loosing compression, I belive the trick to fix it was take a cap full of auto tranny fluid and pour it in through the spark plug, and let it sit over night and that was supposed to usually "unstuck" it. Mine turned out to be that I was missing half a connecting rod. (Never buy motors from shady people) leson learned.

Somebody else chime in on the stuck ring solution, Never hurts to try it, and see.