It would probably be easier to replace the gauge and sending bulb with an original type instead of going electric. One issue with electric gauges is that finding them in 6V is going to be pretty hard.
Most likely the capillary tube to your gauge has been kinked, which would affect the gauge action. Or the needle is physically hanging up. If you can remove the gauge cluster and check that you should be able to either free it up or find a replacement original (try the swap meet here for that.)
