If the ports are closed, you can fill the combustion chamber with Transmission fluid and using a spark plug converted to hold a grease fitting, pressurize the cylinder with a grease gun. This puts equal pressure on the piston so you won’t punch a hole in it. The fluid seeping into the rings should loosen the rust. If you can get it the move a little, you may be able to move it back and forth and loosen the rings.
Good Luck –
