The overdrive seems to have an output for a speedometer hookup. I haven't torn the whole thing apart yet, so I am not 100% sure on that. I ordered a rebuild kit for it, and it only came in a few days ago, so I was waiting before getting into it. I also thought about installing a GPS and forgetting about the existing speedometer. It gets stuck at 65 any way.
I will also have to splice it into the drive line, so at least one new drive shaft will be necessary. I went out to see a drive shaft specialist last week, and he should be able to take care of that.
Intrepid handcycles - http://www.intrepidequipment.com