Another way to get that piston unstuck.
I would like to add more information on how to free up a stuck piston and rings in a 1-cylinder engine.
Normally you let the engine sit for a few days before starting the restoration process. This is the time to soak the piston and cylinder with my mixture of carburetor gum-out and kerosene.
First take the spark plug or the igniter out of the head and put the cylinder with the stuck piston and ring in a vertical position. Next go to the automotive store, and buy a can of carburetor gum-out. Take the carburetor gum-out and mix equal amounts of gum-out with the same amount of kerosene. After mixing well the equal parts together, pour into the cylinder until it is full. Add more each day to keep full, as it will seep down past the piston, which will soften the carbon and rust. Be patient, it will free up. It worked for me and my friends.
You should be able to turn the flywheel in about one week or ten days. If not, then take off the crankcase cover and soak the connecting rod where it fastens on the crankshaft. You may have to take a plastic-tipped hammer and tap from one side to the other to free it. Then take the rod-cap off after you get it loose, but make sure when you put it back together that you put the shims and cap back just like you took them off for a proper fit. You may have to take a piece of crocus-cloth and polish the crank journal some, to get rust and water marks off so it will be smooth, then put it back together, very carefully.
This procedure worked for me twice, so I know it will work with patience. Good luck!