There were, in fact, two problems. Apparently, the timing was coincidental...
I removed the stator windings, and eventually found a break in the "lighting coil" copper wire near where it's soldered to the yellow wire. The copper wire at the point of breakage was completely embedded in a glob of hot-gun resin, so it probably had a tiny fracture, which gave way over time due to vibration and thermal cycling. Anyway, I soldered up the breakage, and it now works fine. Magneto voltages are good. Battery is charging.
I discovered a 50% tap on the ignition coils, so I tried connecting this to the tacho (instead of the 10% tap) -- now there's no bogus drop of indicated revs when toggling the mag switches. Works for me. :-)
Cheers,
Richard
