DC tac hook up..?..

here's someone else seeking to convert a tach drive signal to a electronic tach.
A DC one would be the obvious first choice but other "tach drive pulse signal generator" will probably work.
Mechanical to Electric Tach Drive Adapter
Picture from that thread saved by g**gle


Some other versions here:
Marine Fuel Tank Sending Units for your Boat | Fisheries Supply

Assuming you enjoy a good hunt, this one looks doable.