[Blas]

This gauge is not easily repaired.
I suggest you purchase a new gauge.
Two small knurled nuts on the back side mout it to the dash.
The guage has a thin brass tube wrapped in SS wire that extends to the intake manifild. Use the same routing if you like. No dash removal is required. Small hands are a good thing too.
It's an easily replaced item. Even the manifold connection just unscrews with little or no coolant loss.
When routing the brass tube, be sure it's clear of any potential electrical arc's like sparkplug wires or the distributor cap. Do not kink the tubing.
The only hard part of the job is the dent in your pocket book.