I've seen 3 or 4 of these speedometers where the plastic gear has stripped, as PAT M pointed out it's due to the grease drying out and becoming hard. These speedometers have been ones where the bike has not been used for some time or where the speedo has been unused for a period. The plastic gear should spin freely on the frame but I've never found one that does until the old grease is removed using a small paintbrush and some turps. There's no doubt that the remaining working life of these instruments will be limited due to lack of lubrication. Even if you find a NOS one that will have been sitting on a shelf for years and the grease will have dried out.
I can provide details of the best method to dismantle/reassemble these instruments, how easy it is to do depends on the capabilities of the owner. However if a tiny clip is lost or something is damaged during the process you may find you need to get another instrument and that might not be so easy.
Yohi823 posting 'might be the same bloke', well it just might be! And a 400f at the Gamecock, well mine has been there on quite a few occasions and might just be there on Thursday!