help needed with 904

Today should be hard parts disassembly, cleaning and inspection for any damage.
When you get your rebuild kit it should include clutch discs, steel discs, a front band and gaskets, rubber seals and such, as well as new oil rings where the drums and stuff rode on shafts.
The rear band for reverse is usually reusable. (budget minded) unless the drum it rides on is burned (blue or black) or the material is flaking off.
I like to sand the band a little to break up any polished hard surface on the material.
The valve body probably doesn't need to be disassembled unless a kit is going in it.
It can be flushed and blown out to get the crap from the burned clutches out of the passages.
The valves can be moved with a pocket screwdriver or pick to be sure they move freely.
The pump bushing and seal can be knocked out as you will need to replace those.
Just be a little careful about dinging the inside surface where the pump gears ride.
Red Locktite is a good thing to use on the new bushing when it goes in to insure it does not move.
When you are ready to put the new bushing in we will talk about staking the bushing.

These are just FYI's which I will be giving you to keep in mind as you go.
Also keep in mind it has been a few decades since I did these for a living, so I'm not going to be perfect.
Others may chime in with things I haven't mentioned yet or forgot about.