You are right to ask yourself if you can work on the T-89J transmission. For the most part it is very easy. It doesn’t contain so many parts that you can’t do it yourself.
At a minimum you would normally want to replace the bearings at each end, gaskets and the small parts kit, oh, and perhaps the cluster gear shaft. However, if there is rust everywhere on the gears, you may find it necessary to replace even more. If you read the TM 9-1803B, it will help you by providing required tolerances. The last transmission I worked on had parts supplied by Richard Grace. His prices were very reasonable. I believe that all the parts come to under $600 (2003 prices). I’m not sure he is still in business. But you can get a rebuilt tranny for the price of the parts? Right? Well, apparently the secret is that you aren’t getting all new gears only what’s “necessary”. So for the same amount of money you will know what you have. Of course, if you mike the gears for tolerances you may be able to get off much cheaper.
The two screws are likely bristol types.
Though apparently other types have been used since the war. If you have already mucked them up as I had done, I used a 3 piece tool set from Sears that is designed to remove mucked up screw heads. No amount of fooling around with the mucked up screws allowed me to get them out…but less than 30 seconds with the Sears tool and I was done.
I wrote a book about the T-84J transmission called Trouble Shooting And Rebuilding The T-84J that you might find helpful. It goes into more detail with a lot more pictures than in the military manual.