If you can find an old voltmeter with that type of meter movement, or any other galvanometer movement, you may be able to disassemble the tach movement and replace the tach coil and axle with that from the voltmeter/galvanometer. You have to check the coil resistance to get something pretty close, but I think you can find one and make it work. The risk that remains is that the bearings will be badly corroded, so you may not get smooth needle movement, even by replacing the coil and axles. But the existing axles may be hard enough that they can be cleaned up. If you are good at soldering and have steady hands, you can probably make it work.
Harvester
