Not sure if it can be of any use still, but here's my input:
I'm using a 52mm diameter electronic dial tachometer (must be capable for 2 cylinder engines). Pricing was around $25,-. You do need 12V. I installed a small 12V 2.9Ah battery, which is easily connected to an external battery charger. 12V + and - go to the tachometer and signal line is connected to the killswitch wire.