The use of an optical tacho is a standard technique, cockpit tachos often misread very badly.
Any Rotax engine dealer should carry them, or a model aircraft suppliers - aeromodellers use them a lot.
If it is wrong, there's not much you can do to correct the problem, just make a note in the logbooks and placard the correct (indicated) RPM limitations and cruise settings.
Incidentally, if you have a Rotax or similar engine which uses a gearbox, you need to multiply the propeller RPM by the gearbox ratio to get the engine ratio, or it'll never make sense.
G