Hello
There is no simple hook up to get the tacho to work.
The petrol tacho works by counting the ignition pulses.
If you were really keen to keep that tacho you could fabricate some sort of pulse to happen off your crankshaft using a hall effect or a magnetic pickup like a crank angle sensor. It will take a bit of fabrication but you have already done the hard work.
Your local electronics hobbyists shop might be able to help you with some components for a pickup.
regards
Mechanic Martyn
A grade Mechanic
Advanced trade certificate in Automotive engineering. 30+ years in the automotive industry