Coils can fail when they are hot because the windings expand with heat and if there is a bad spot in the insulation it will fail, but that is usually a thing that happens while driving, but it could be the culprit. I would look closer at fuel though. Unleaded gas is much more volatile than the old leaded gas these critters were built for. Do you have an insulator between the carb and the manifold? If so, I sometimes stack a few extra gaskets to keep the carb from perculating. Try holding the throttle wide open when you crank. This will give it some air if it is flooding. Also a dragging starter will take voltage away from the ignition. I make sure my starter bushings are good and I install high performance in my starters. My 6 volts crank almost as good as 12 volt.