Later model electronic programmable VDO tachs will read anything ................. flywheel output, alternator output, distributor output (gasoline), heartbeat and heart pacemakers will provide a useable signal. Pacemakers can be problematic if a pretty girl walks by.
The tach I installed about ten years ago is the VDO Vision 333-163. You can set it up by calculation but it takes only seconds to set it up and calibrate with a cheap optical tachometer. Be careful that the VDO face matches the hole left behind by the old tach. If so, there are other makes of tachs which may fit.
if you don't have an extra tachometer for calibration, set the VDO tachometer to pulse mode ,then set p=97. That would be OK. The flywheel of Yanmar 3GM given 97 pluse per rev.to the sensor then to the tachometer as reference. or you can go to the adj mode to cal by an individual tachometer . Yanmar has a technical bulletin no. MSN 03 -015 telling the procedure for PULSE mode setting