I recently bought a NOS Stewart Warner speedo for my '54 Ranch Wagon from Speedometer Service in Colorado Springs, Co
https://www.speedoservice.com/
Rusty went through it before shipping it to me since it had been sitting on the shelf for a while. Can't say enough good things about them. If he doesn't have one to sell you I'm sure he can repair the one you have. Worth a phone call, IMO.
449 E Kiowa St, Colorado Springs, Colorado 80903
(719) 632-3165