[Dave at Pelican Parts]

I'm pretty sure that the white-backed gauges are aftermarket. IIRC, all of the original ones have metal "cans" that serve as the body of the gauge. I don't know what would be checked inside the gauge, but I would not be surprised if just a clean and re-lubrication (with much lighter-weight grease than you'd generally find in a car!!) would get it running again.

Though that assumes that the clock is mostly mechanical, as I think the original ones are.

See if you can find part numbers on the gauge and search around for instructions for that part number.

--DD