The transistor radio for mag timing is really cool. Here's another
simple idea:

Drive the magneto from a 50 ohm output audio oscillator. The resonant
frequency of the coil and capacitor is around 1kHz. You can hear
the note from the mag coils.

When the points are open, you can tune for resonance by listening. It
will be the loudest then.

When the points are closed, the note will go away.

Takes an oscillator that will do about 5v into 50 ohms or you will have
trouble hearing it.

Bill Hale