Open it up
This is nothing new to us Airheads.... I would open the old set up and see what is going on. A lot of pieces are plastic and after some time give up or jump out of place. It may be a simple fix. You will not know until you open it up. Any Airhead speedo will work, but you will have to calculate the differences based on your stock set up (speedo and rear drive) and what you are replacing it with... You may be able to get a used speedo and and harvest the parts and fix yours...