Seat switch sounds bad or wiring is faulty to it. If you sit on the seat it should open the circuit allowing the ignition coil to fire. When you get off the seat, seat switch should short out or go 0 Ohms and kill the ignition/engine. Think all the safety switches are wired in series. I'd have to check the schematic on that one. Some of the others should chime in with more concrete answers. If you ground out the coil wire, it should kill the ignition to the spark plug.
slomo