I fixed a guys moped that had a jug once. Can’t remember what it was but the piston was stuck.
After soaking, like you are doing, I looked in and saw the piston was below TDC but not too far. So, I stuck a wood dowel rod in the spark plug hole and beat it with a hammer until the piston broke free.
I also used electrolysis to remove rust from the tank. 1st time trying that. Made a nasty mess but was fun.
He had his moped back in a week and used it for moving his ass around the drag strip on race days. Guess all that banging didn’t mess it up too bad.
I would never do that for myself or to sell something but it’s what he asked me to do. Just get it to run. Proved a point about how resilient a stock moped can be.
Sorry for the long story. I maaaaay be drunk.