Circuit for finding engine RPM using the magneto's coil ground wire
A simpler approach is to make a crude optoisolator using a phototransistor and a neon tube. enclose both inside a plastic camera film canister to shield from external light sources. 3 or 4 turns around the high voltage lead going to one lead of the neon bulb, the other goes neon bulb lead is grounded. Cheap, easy, not very time consuming and gives your tach circuit a lot of high voltage isolation.