If the speedo is a Veglia, Palo Alto Speedo advertises they repair those. Check the tiny print on the face to see. If another brand, post that here and I'll help you find a place.
Repairs are not cheap. Likely 200 plus is my guesstimation based on getting some BMW gauges done.
If it's not in the budget now, get a decent bicycle speedo.
I put one on my Z1 race bike just for kicks and my GPS said it was crazy accurate. Went to the AHRMA races at Big Willow and the memory stored a top speed of 121 on the back straight before the big turn. Gulp. I took it off.
Old bike, skinny tires, old fat rider, ignorance is better sometimes.