Thanks for that, KRV. Unfortunately, it doesn't help - the lower part of that pic is just about what I have been trying but with a new coil in its place. The primary current is something over 10 Amps (the books all say 1.5 to 2 Amps), and the coil becomes untouchably hot in a few seconds.
The type of magneto-coil depicted had no electronics in it, it was a simple transformer in effect.
The coil I have never went even close to the engine - it's mounted on the chassis, somewhere under the saddle - so it never gets a magnetic pulse, either as power or as a trigger.
The guts of the coil I broke-up clearly contains a circuit board with some resistors, a large capacitor and a power-transistor, all of which suggest the thing should be polarity-sensitive. But which polarity? My Honda motorbike's manual gives no indication.
I'm guessing that both power AND control-pulse must come in to the thinner wire - the fat one is HT to the plug - and there was a second, orange lead riveted to the core, which I assumed to be ground/chassis return to the battery.
Geoff