This is not a common speedometer failure mode, it is usually the odometer that fails. I expect that your best bet is a salvage yard speedometer. It is easy to replace by pulling the cluster and changing it from the back.

If you want to duplicate the odometer setting, file off the swage around the perimeter of the odometer shaft, carefully pull out the shaft, set the odometer to the correct mileage. Then put the shaft back in and brad it again so it won't come out.

Good luck,