A typical automotive tach only has settings for 4, 6, or 8 cylinders. You need to be able to set it to 1 cylinder if you have two coils (points, re-phase Pamco) or set it to 2 cylinders if you have a single coil (TCI, Pamco, Boyer). You may be able to find a tach with these settings but more likely you will need to bust it open and recalibrate it.
Older style tachs used analog circuitry. The selector switch simply swapped in different value resistors to change the calibration. You can look at what resistor values they use for each setting and approximate what value you need. Or better yet, swap in a trim pot so you can dial in a perfect calibration.
The newer tachs use digital circuitry which actually makes it easier to tweak. I did one for my DRZ400 and the IC had four input pins that told it, in binary, the number of cylinders. So if you want it set for one cylinder you would put power to only pin-1. For two cylinders you would put power to only pin-2.
You don't mention what ignition system you have but chances are the tach will have no trouble reading it. All the tachs I've seen are made for inductive ignitions. Points, TCI, Pamco, and Boyer are all inductive ignitions.
