Zenoah makes some fine pieces of equipment but their cutoff switch is not one of those fine pieces.
Just go down to the nearest Radio Shack and buy a heavy duty toggle switch to replace the Zenoah switch. I use the heavy duty switch simply because it handles vibration better and has a big enough handle on it to easily shut off the ignition quickly when needed. A slide switch is probably more reliable, but not as handy to use.

Building is easy.------ Maintenance is a chore.