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Bike has a mag (no battery in the system) and has always run well. Points ignition.
Recently, when it gets good and hot, the ignition stops working (bike dies). Let her cool down and she starts right up again and runs fine.
Replaced condenser with a new, externally mounted unit--no change.
So, it's either the coil (mounted under the tank) or the stator/mag unit in the case.
Everything in the stator is original, but I did have the flywheel re-magnetized about 15 years ago.

Advice appreciated.

I would do the coil next.They do heat up with use and vibration over the many years will take its toll on the windings.Plus it is easier to get to than the magneto stator.

Yes, it's almost certainly the ignition coil. I have a new one on order. For those of you with magneto-equipped singles, one coil which works very well is the 6V AC coil used on many of the old Honda singles. The coils are easily identified by the words: AC 6V embossed on the coil, and by the fact that they have only one wire. The coil grounds through its mount and the single wire connects to the condenser/points/hot AC wire from the magneto.