motor rpm activated servo for CVT in Ebike.

I have a idea to use a 50cc chinese pocketbike cvt from the popular x2. Though instead of using a weight setup for consistent motor rpm I want to use a servo that presses on the same plate the weights would at above a preset motor rpm. The gearing up, so to speak, would lower the motor rpm causing the servo to stop and stay put until the bike and motor went faster and repeat and repeat. Conversely the servo would have to be reversed when the engine slows below a specific rpm so as to reset it when you come to a stop. I'm know more than the average joe about electronics but I don't know what type of circuit I would need to make a rpm activated switch.
Thats my question. How do I make a rpm activated switch???
Thank you. -Ryan