The problem all depends on what isn't working. Is it just the speedometer? or the odometer too?
If neither are working you can possibly fix it as it may be a busted cable or something else in the gearing very soon after it enters the housing.
If it's just the speedo, you have a paperweight. That is, unless you can do watch spring vibrating. The speedo, if it's like all the others of the era, uses a magnetic coupling. One side of the magnetic coupling is connected to the drive cable, the other side to the needle via a spring very similar to the balance spring in a non-pendulum clock or watch. The tension against that watch spring, through the magnetic coupling, is what displays as your speed. Super delicate stuff, and very easy to damage that watch spring or the bell housing of the magnet. If you spun the needle at all, or played with it, chances are that spring is fubar.