This is typical when the compression gets low due to worn piston/sleeve. When cold, the fit between the piston and sleeve is tighter. When the engine is new this will make it somewhat difficult to turn the engine over top dead center. At operating temperature, the ABC construction will have expanded to produce the correct fit.

So now that your engine is old and tired, the fit is decent enough when cold to provide ok compression and let the engine start. But after things warm up and expand your fit is too loose, compression is down, and it wont run as well. Try to restart it when warm it it probably wont start at all. Let things cool down and it will restart again.

Sound about right? It could be other things, but what you are saying sounds pretty familiar. You can get a new piston and sleeve and just swap that out. Depending on the engine, it might be half the cost of a new engine. Or if you were looking for some excuse to put a huge new engine in there and blow up your transmission gears, this could be your lucky break!

For what it is worth,
Spiro