The magneto coil is about a riot to change. Did one in the field by removing the magneto and somehow got it all back together with everything in the right position and timed right.
There are ways to get around it I have not tried , like adding an external coil.
There's been mention of a wet rag being placed on top of the housing to act like a heat sink in the past.
Just the location might factor in it getting heated so much near the engine. Combined with the mass not dissipating heat efficiently once heated.
I've conked out a couple coils working hard in the heat.
Last time I just left it in the field ,came back the next day and it ran. But is it weaker now?
The external coil ignition on the other Cub has behaved just fine.
Both are 6 volt.
Lots of good engineering in them Cubs. But like most things , there are weaknesses too.
Your friend with a brain might be on to something.
