Pat,
you need to find a "bevel gear" set. Final drives are on the sides between the steering clutches and the sprockets.

I don't have any parts book to help you , but I was not aware that any of the old CAT tractors had anything other than straight cut bevel gear sets.

You would probably be better off to replace the gears in a set, but you might get by, with careful adjustment, just finding a ring gear ( the large one, also called a crown gear by some folks). I blieve that replacing the pinion gear would involve some disassembly of the transmission beyond that necessary to set up the bevel gear set.

Maybe the gears will interchange with the later tractors.

From looking at the pic of the rear end and what's in it sure makes me think that the ring gear (along with the whole cross shaft, brakes, drums and clutch disks) would interchange with the J and U models IF the tooth count on the ring gear of all 'em were the same. I've found quite a few things that do interchange between the J and U models but have differing part numbers. This might be the case here, but don't take my word for it. It's my understanding that from the time Cat went diesel each of the subsequent models were an improvement over the previous model, so I'd think some of the different serial numbers might be a change in metallurgy or heat treatment rather than a complete redesign.

I'm by no means an expert on these critters so if someone else knows, don't hesitate to pass along the correct knowledge.