When hottest immediately pull a plug and test for spark. A failed ignition module will work at cold, fail at hot. What are your compression readings, especially before and after gasket replacement and cylinder vs cylinder.
Well guess what I fixed part of the issue -- new carb --- adjusted the valves again !!!! --- starts and runs great ---- cut the middle 40 acres 3 times - no issues - just put in a new ignition switch - now the darn thing will not shut off in the off position %&*(&^%%$#$#@
That will be the ground wire on the coil fallen off or no ground connection to the ignition switch.
On many switches the ground is a stand alone terminal and not on the plug.
If the engine has fuel solenoid that should also cause the engine to shut down from lack of fuel.
Because I am a belts & braces man I like to fit a fuel valve on the fuel line.