For a start, I suggest you check that the engone coil is OK. Find the magneto ground wire (usually connected to a terminal near the throttle linkage) and disconnect it. If you still have no spark, you have a problem with the coil - forget the rest of the wiring. If you DO have a spark, but no start, check that there is battery voltage on the wire leading to the carb float chamber. This is the cut-off solenoid, and cuts the flow of fuel to the main jet if not energised. If there is battery voltage here, you have sparks, and it still won't run, change the fuel solenoid.
Although possible, it's unlikely for the ignition switch to go faulty, as it would have to fail CLOSED to cut the engine. Since it was running when it died, the switch would have had to fail spontaneously - from open to closed - without being touched. Not likely.
phil_saunders@bigfoot.com
Phil Saunders, August 2008