You'll need to be more specific to determine model. Year of manufacture, 2 or 3 cylinder, serial number, etc.
Look here:
https://www.boats.net/catalog/mercur...ard-by-hp/30hp
Under some of the models and accessories sections - some came with optional tachometer and harness. Might not necessarily be available new, but once you have a code, then easier to track one down second hand.
Hopefully another member can provide maximum RPM if you provide information on the engine.
Sula
