If its at least a twin cylinder and not a single cylinder, around here on old tractors, they take the valve cover off and see what cylinder has the valves closed and remove that spark plug, fit a hydraulic adapter to the spark plug hole, hook the hose to a tractor hydraulics and use hydraulics to bust them loose, slickest thing I've ever seen done, and if it won't go, chances are, it wasn't rebuildable anyhow.
I've been told, on a single cylinder engine, you remove the rockers assembly so the valves go shut, then use hydraulics to bust the cylinder loose.
This only works as long as the piston is not at the bottom dead center, anywhere else in the stroke it will work, filling the bore with hydraulic oil and using pressure to shove the piston down enough to crack it loose.
As they say, you asked for idea's, might be a bit radical, but there's a place around here that buys old tractors, and does this by the hundreds, maybe even thousands of engines to break them loose over the years, if they blow the engine wasn't worth fixing anyhow. Just a suggestion and off the wall method not mentioned.