The speedo driven gears are usually dealer items, but they're getting harder and harder to find. They're color coded for the number of teeth, so if you can take the old one into a dealer and talk to an old-timer in the parts department, he/she might be able to get you what you need.
But beware: if the teeth are gone on the driven gear, they're probably gone on the drive gear. On a Saginaw (as I recall) that's on the main output shaft and is the last part to come out during disassembly.
If all else fails,
this company may be able to help you out.
Good luck with it,
