Sun tach rebuild

You'll have to do a Google search. There are numerous tach circuits over the years DIY and otherwise, and there are people making tach replacement boards for such as the old Mopar factory tachs

The "tach head" is simply a meter. I don't remember the "full scale" ma rating. If you can find a tach with a busted meter you might even adapt the guts to your meter. DO NOT however try to use a modern one such as Autometer. The clue here is that some modern tachs the meter movement does not "fall to zero." The needle stops and stays/ floats wherever the key was shut off. These are a different setup