The Elusive Speedo Input Gear!

I'd almost bet money that the issue is the box between the two cables rather than the drive.

Those things are notorious for making needle bounce and all sorts of other silly things. Many people have just gone ahead and gotten the one piece cable meant for an S2, installed it, and been much happier ever since. (I know i wasn't dissapointed when I did it on mine. Not to mention the single cable is cheaper as a whole than either one of the two pieces for the original setup)

If you uncouple the trans cable at that odometer box and cannot turn the cable by hand, then the drive gear is most likely in good shape.

However, if you do end up needing the gear and none of the regular sources can get it, chances are pretty good that if you give APE a call they can get you a really good condition used one.

Oh, and to actually answer your question about the gear:

The cable drive gear can be removed without messing with the transmission, the gear that drives that one requires opening things up.

So what does that box do anyway? I've noticed it's got some wires coming from it. I can simply install a cable from an s2 and away i go?

Yes, away you go.

The box turns on a maintenance reminder light every 30k miles.