If the engine is not firing, valve clearances shouldn't be of concern until it is firing. You can check the compression with a gauge while cold, then when hot, to see if a valve gets lifted when hot. Coils are prone to heat related failure, causing problems that can resolve when they cool. As they expand and contract with the temperature, damaged insulation can cause them to short out, or an internal broken wire can open the circuit, etc. Engine heat transfers from higher to lower temperature areas when the engine is stopped because the air flow stops.
I've had trouble with a flooding carburetor when the engine was stopped. The carb is between and above the cylinders and heats up when the engine is stopped, this caused the gasoline to boil over into the intake manifold. Check the plugs to see if they are wet when it doesn't start when hot. After it sits a few minutes when hot, look for fuel puddling in the intake manifold using a light, by looking through the carburetor w/ the choke and throttle wide open. I lowered the float level slightly to resolve that problem. But again, not a lack of spark issue. Systematic troubleshooting helps pinpoint causes for problems.
