I'm running V-twin motors which have engine speed governors on them but am wanting to setup an electronic ignition cutoff at a set RPM as a failsafe so if a governor fails the engine won't self destruct from being over-revved. I've found several after market ignition control modules made for 4, 6, and 8 cylinder motors but not much for V-twins. I'd need to setup about 50 motors on our fleet this way so don't really want to shell out several hundred dollars for each buggy just to kill the spark above a certain RPM. We run our motors at a fairly low speed and would need to cut spark somewhere between 4,000 and 5,000 RPM.

any ideas for a cheap rpm cutout circuit?

The GY6 150cc singles have some Capacitive Discharge Ignitions (CDIs) available with adjustable advance and rev limits. I'm not sure if your v-twins have an equivalent or if the GY6 units could be converted for your use...

You may want to check resources from the track karting community to see if something exists that would plug in with out modification or look at industrial engine governors that may be more reliable.