I don't think you want to get the voltage out of the ignition coil. You can sense the pulses through the spark plug wires. You could build an EM sensor that goes around the spark plug wire. Everytime it pulses, read it and you will know your RPMs. It would be have to be very sensitive. I remember reading the pulse on a oscilloscope and you could see it, but you could also pick up the pulses on each spark plug.