Marine Engine Synchronizer guage

[Spendandfix]

Could someone share a simple circuit diagram to build a marine engine synchroniser indicator? I believe how it would work is to measure and compare the frequency from each engine ignition coil and use three LEDs to indicate whether the port engine is too slow, (left LED is lit) the starboard engine is too slow (right LED is lit), or both are matched (center LED is lit.)

I have very limited experience with electronics, but am capable of building a simple circuit if one is available. I just purchased an Arduino beginners kit to strat playing around.

Thanks.

Jim