The ignition coil went bad. Unfortunately you can't replace just it. They put it inside of the ignition module.

I have had the COP coils (coil on plug) go bad and behave exactly like that. The engine would idle and run up some RPMs and then flake out. But since the coil was outside of the ignition module I could measure its coil resistances and see that it had gone bad. The model airplane CDI units are all sealed up and potted with epoxy, so one cannot just open them up and check or measure anything. Even on the few that they didn't use epoxy to seal them up good with, if you try and take them apart they sort of get all mangled in the process.

One cause for the coil to go bad is an excessive spark gap.Where the spark has to jump a distance greater than the small gap on the spark plug. That large air gap causes the voltage levels to increase or spike tremendously. A example is looking to see how long of a spark one can get, or quite common is the spark plug cap coming loose on the spark plug. Most modern cars state to never ever pull a coil off of the spark plug as it will destroy the coil immediately. usually our model engine ignition systems tend to have a bit of a delay effect when the coil has that happen to it. The coil goes bad later on you.

The COP (coil on plug) design is where they use a higher voltage to drive the coil with. Such as 80 volts or more. The CDI system has a voltage multiplier circuit inside of it that boosts the battery voltage up to some pretty high levels. The higher the voltage you use to drive the coil with the smaller you can make the coil in size. The COP coil does not have to be mounted right on top of the spark plug, but that is the more ideal location for it, all the car engines have pretty much gone to that method as they eliminate the need for a distributor then. So the small coil can be separate and still use a spark plug wire. Since the model engine CDI modules are sealed up and potted, I can't measure any of them to see what the voltages are that are generated inside of them. But they do generate a pretty healthy spark across the spark plug though.