I have converted several gauges (water temperature, tachometer) to electronic use with more modern electronic fuel injected engines by replacing the "guts" of these gauges with modern gauges from Autometer. Sounds like you may have done the same. The speedometers/odometers are more difficult to convert so I use a device called Cable-X to convert an electronic signal to a cable drive. The Cable-X is rather expensive but there are other, less expensive, devices that do the same thing. You should be able to drive your old Smiths tachometer with one of these units.