Speedo and cable spent, questions

yes you can drive without the speedo and cable. I would think the drive gear and seals are fine.
The speedometer head has a bronze driveshaft in a potmetal bore. There is a oil hole with a fiber wick under a little cap. If the cable pushes on that shaft it will overheat and sieze up. You can remove the cap and wick then flush the bore with 3 in 1 oil , while turning the shaft with pliers. Maybe it will loosen up and live again, maybe not.
If I bought a used speedo off ebay I would do the same things to it before install too.
Servicing the rallye panel is such a chore :( A restored speedometer is no doubt the best route in my opinion but... You could break the bezel, a circuit board , whatever, while intalling any speedometer. DOH ! happens
Good luck with it.