If the speedo works after mild percussive maintenance is applied to the dash above it, the problem is likely some oxidation on the contacts where the lead from the rear axle speed sendor connects on the back of the speedo.

The little gear that breaks is brittle with age, a swap may not be a long-term cure. There is a place where the gear may be purchased separately. It's been so long since I bought two that I cannot find the place. I know there is a speedo repair shop in Austin, TX and, hopefully, another brickster will provide the contact info.

Good Luck,

Bob

:>)