Here is a picture of my 80's tranmission. Is the arrow pointing to the gear you need?
That is the R1492R Differential Drive pinion. As you can see, mine is also pretty chewed up. I managed to find a replacement (though I haven't swapped it out yet-BIG job). I had pretty good luck finding this gear through this website's Classified section. You should try it. You'll most likely only find good used. I doubt there are any new gears on the planet.
By the way, I've heard that this gear usually gets most of the wear. Has anyone else experienced this?
My Industrial John Deere website
John Deere Diesels
The LaGrange Engine Club