904 Transmission Stopped Going Into Gear

The parts for these transmissions are not difficult to locate, and it does not sound like this is your primary ride, so I would suggest doing the tests and checking the numbers stamped into the case to determine what you have to work with, but waiting to order parts until you get it apart.
If it had to be back together in one weekend, I would say order a stock rebuild kit and converter to have ready to go.