Many of the very simple old 2-strokes just relied on the magnet
*leaving* the field to time the spark. Sort of time, that is. g As
the motor speeds up, the spark timing actually can advance a few
degrees that way.

You may be able to tell from the relative position of the magnet in
the flywheel, to the magneto coil, where the timing is. In any case,
it will give you guidance about setting it on the opposite side of TDC
if you want to run the motor in reverse.
I have worked on a lot of old two stroke motors, motors with cast iron
pistons, completely unbalanced cranks, and grease cups to lubricate
and seal the crankshaft where it exits the crankcase. And I have never
seen an ignition like that. The oldest magnetos I have seen had points
of some type. I can't see how leaving the magnetic field would cause a
spark. Can you explain it?
Eric