Speedo gear broke inside tranny?

The O-ring is necessary and if you think there is something floating around in the xmission I would get it outta there ASAP. Been a while and depending on what transmission you have I think all you have to do is remove the tail shaft to access the speedo gear.

What transmission do you have?

If you've got a C-4 then just remove the tailshaft housing, I don't think there's any other critical parts back there, that would be harmed by a plastic gear. The drive gear is machined into the tailshaft.

place a block under the trans pan, jack it up slightly. Remove the driveshaft ( be prepared for the flood of ATF when removing the driveshaft slip yoke ). Next remove the nuts holding the trans mount in place. You may then remove the trans cross member at this time to gain more work space. Otherwise unbolt the tailshaft housing from the main case ( be prepared for another flood of ATF ) After removing all this, you'll see the speedo drive grooves in the tailshaft. If they're chewed up, then you're screwed as far as a working speedo. Otherwise dump the speedo gear remains out of the housing. Be a good time to change the rear seal at this time and check the U joints on the drive shaft too. Use a new gasket to reinstall the T/S housing. Then reassemble the whole thing in reverse order. Be sure to add more ATF before operating the car.