Trial and error works well for tachometers. They are mostly just three phase generators at one end and three phase motors in the meter. The engine driven generators usually make about 20v ac, and I have managed to get them to drive virtually every tacho that i have used. I only get them to about 3000 rpm as they make an awful lot of noise in operation and i am a bit delicate with such old stuff.
I even made a circuit that drives the meters that can control the indicated frequency although the waveform is not too clean and i only really get it 4000 rpm.
I have only ever bought one tachometer that did not work. They are pretty simple and robust items.
Gary Morton