Call around and get estimates based on what you're describing. The big three are: Nisonger, Palo Alto Speedometer, and APT (gaugeguys.com). Describe what's happening with your gauge and they may be able to give you estimates over the phone.
However, depending on which vintage speedo you've got, you could try minor cleaning and work on it yourself. Google for the 27 page PDF by Anthony Rhodes entitled "Repairing Jaeger and Smiths Speedometers". Armed with that, a digital camera, and a clean place to work you might just be surprised what you can accomplish.