If you can't find any new, LM2907s are readily available from old 5.25-inch floppies as the spindle drive motor tach circuit.
If you're going to go to all the trouble to detect the RPMs of both engines and still have to do some manual tweaking on the throttles to make them match, why not go all the way, set the speed with the starboard engine, set the port engine close and then flip a switch to "AUTO" and let the output from the starboard tach adjust the throttle of the port engine to match? Hey, if it's going to be a neat project, why not make it an insanely COOL project to boot?
Dean