Good work! These 900's are nice tractors - I'm in the same positions as you with the brake drum missing, I actually stopped using the tractor because of this. I'm afraid to take the transmission cover off to look at the gears.
Do you have a trick to remove these old rusted exhaust elbows without breaking them?
Yeah, I dont know how they ever left the factory without brake drums, especially when they used them on the husky 600's already!
The tranny's are usually pretty ugly on the models without the brakes, the gears in mine are basically new as my Grandfather replaced them in the 90's while they were still available through Bolens. Hopefully your previous owner stopped fully before shifting.
Most of the time you can get a pipe wrench on them and do a couple taps with a hammer and they will break free, if not dont press your luck, if its real rusty just cut the elbow right off leaving about 1/2'' sticking out the block, then take a hacksaw blade and cut the remaining bit of pipe just above the threads, take a cold chisel and punch in the pipe where you cut it and it will either break free and you can unthread or if not make another cut and try again
Someone made a good thread with pics on how to do this method.