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Used to restore old one-cylinder farm engines -- often stuck. You are doing the right things but you are trying to bust everything loose at once. Take out the crank and periodically smack each piston with a piece of wood and big hammer. Some will come loose sooner than others.
Douse the valves, guides and lifters with PB Blaster and tap each one with a soft hammer.
Once something starts to move squirt it and tap it in and out. If it moves it's on the way out.
The last engine I restored (check video below) had deep grooves in the cylinder and a lot of rust. I hit it with a hone and put it back together. "Clearance" meant something else in those days. The engine is 6 hp and weighs 1,860 pounds! Good luck.
https://www.youtube.com/watch?v=fLB5BxO8GvA