What is wrong with it? if it is just the shift fork on the lid easy fix, you seem to know your way around this stuff better than i so guessing it is more than that.
The books are good to look at, i printed it off and even glanced at it a few times. Probably should have read it a bit better but those parts did not make sense till afterwards.
If the tranny needs to be pulled and everything else is good(aka the hood was not removed for a decade degrading wire connectors, hoses and letting water into things that should not have water) it really is not too bad with two people and something to lift with. I would not care to do it solo.
Remove the loader and backhoe. I had to use another tractor as my see would not run, so no tips first hand other that backhoe is
heavy, i had a tractor on each side to lift it and towed the see out from under it. Second hand, get a jack for a trailer and make an adapter for the square tube on the loader. On the back hoe unhook as you see fit, when done chain the outriggers together so that it can not settle.
Remove hard hat with tractor.
Remove cab, fairly straight forward alternator needs moving in. Unhook everything in site. The driver side is a whole lot heaver than passager side, chain with that in mind. Also figure out the brake cable before you start, mine was cut as we got to a place where that was all that was left and no obvious way to unhook it. When lifting take care of the fuel return hose.
Nothing exciting on the engine other than the oil well on the bottom, the big thing on the bottom of the pan. Plan on a way to either leave the engine in the air or have a plan for blocks, i used old tires, as you do not want to just set it down. Having a floorjack under the tranny is likely a good idea as you can adjust some of the angles by raising and lowering it.
That gets you down to the tranny. Front axle needs pulling and moving forward to get acess to the bolts on the front torque tube. Note the two missing bolts on my tranny you will need to pull them otherwise as expected on the front. Remove the back driveline and anything else and it comes out. Check the boots on the tubes, now is your best chance to change them.
Having done it once i think i could pull a second tranny in a short day with a helper and tractor that does not settle.
Oh have complete metric and sae sets of tools. While there is a pattern to it, at first glance it is almost random what fitting is where. A decent set of metric bolts will help when putting back together.
Send me a pm if you want to call and chat some evening.
As for fix vs part, if i was you i would fix. you are starting off with a lot better tool and at least on the west coast looking at 20k+ for one that runs. over 30k from what you are describing if you find the right buyer.
I payed 6,650 for a rebuilt engine and less than 3k for a mystery box tranny that appears good. Could you get 10k for the rest of the parts, probably but it would take a bit. If you need tranny parts look me up with luck maybe what you need is one of the few not rusty ones in mine.
On a selfish note if you do part out i am looking for a few parts such as a passanger door, that i rather buy vs make.
Good luck and keep us in the loop.