get a peice of metal, thin stuff, cut it and bend it and drill it so the spark plug can pass thru the hole, then bend the other side around so if you press on it, it will contact the spark plug wire... coat the top w/ spray undercoating, to make sure you won't get zapped... B&S engines did this for many years on go kart engines.

for a switch
It's fairly easy, your key switch will have continuity between 2 terminals in the off position, it will go away in the run position.. on a B&S engine, the mag shut down wire is black, and will run under the flywheel cover to the magneto


however if you have continuity in the on position and not in the off position, you don't have a magneto, it's a coil operated ignition.