> But AFAIK there IS no "low tension side of the magneto". The spark is

The magnet passing the coil is not enough to make a spark. A normal
magneto has a pair of points to break the circuit when the magnet passes
the coil and generates maximum field (and thus current).

There are very small engines that do not have a separate HV coil. The
ignition assembly looks like a tiny wedge of cheese with a HV wire to the
spark plug. This is screwed with two (or more) machine screws onto the
engine block facing a magnet on the crankshaft. The Holes are oval for
advance adjustment. Here they use a Hall effect sensor or a dB/dt coil and
an electronic switch buried in the main coil to do this. I suspect you
have this kind of engine.

In this case, try to find the ground lug of the assembly and put a relay
between that and chassis. Adding a set of windings around the wedge and
shorting them to stop will also work.

Peter

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