ignition pickups are readily available from automotive supply places they are used on cruse controls to prevent engine runaway in the clutch is actuated. the true speed is taken from the driveshaft or the speedometer signal depending on the vehicle. a second source is replacement pickups for timing lights. since both are electricaly isolated pulse pickups operating in a relativly constant enviroment it should be easy to condition the signal and clamp it to protect the electronics then if you want more protection isolate it with an opto coupler
Interesting project Terry