There is no need for any external connections to the magneto unless/until you want to stop the engine, which in the end is important, particularly if the machine bolts across the lawn toward your neighbour's cat. If you had a pull-start it would be easier to test various things, but you might try something like this. Arrange the wire coming from the magneto's only external terminal (i.e. the kill wire) so you can ground it easily but in the meantime it won't get wound around anything and strangle somebody. Clamp the metal base of a known good spark plug to a cylinder head fin, and connect the plug wire to it. With the kill wire not grounded, operate the starter and watch the spark plug gap. You should expect to see an even series of blue sparks.
