You could search ebay for "instrument voltage regulator" and find several examples with a wide range in price. The optimal solid state instrument regulator comes from Real Time Engineering. It has LED indicators but more importantly It has ground fault protection built into its circuitry. Of course it is the most expensive too.
Any alternate regulator will need to be mounted somewhere under the dash. You will disable the original regulator and route wiring from the new regulator location to the instrument panel.
With internet research you will find pictures and info about opening your fuel gauge to disable that internal regulator but that isn't necessary. If a 3 volt battery properly attached to fuel gauge or temp gauge moves their needle to about half the range, no need to open either of them.
Post a pic of the back of your inst' panel. Remove your fuel gauge and post a pic of its backside. I can cover all the details.
