any 2 cylinder rev counter will do, a Harley sportster one is perfect, its used to vibration and low power out puts hehehehehe
seriously tho, it has a red line on 5500 rpm, which is spot on for an aircooled,and if you tune the motor, you can take it into the red....very satisfiying.....ive seen 7k rpm in anger with my mildly tuned 652cc !!......they do pop up on ebay from time to time...
i mounted mine in a (empty) boduelle bean can...then made a brkt to mount it above and to the left of the dash pod.....
the guy who has my ex blue buzzbomb 2 will have the wiring info inside the haynes manual i gave him, i wrote it inside the front cover.....
its a worthwhile mod...