We seem to have less electronics expertise than in the past, what
hppened to Andrew Gabriel?

I have one of the cheap 2T 650W gensets based on the Yamaha 50cc
capacitively excited generator handed me by a mate to fix, a NUTool .

It won't start because the HT is too weak to produce a spark and I
currently put that down to loss of magnetism in the flywheel magnets.

These things are cheap low quality copies and generally not worth
repairing but I also have my first machine saw from 1974 that exhibits
the same problem. They are both capacitor discharge ignition.

Any suggestions for a simple battery powered circuit which could sense
the trailing edge of the pulse to the coil circuit I and use that to cut
the current running through the coil and produce a decent spark?