Wow, the last time I worked on a System 25 I think there were still dinosaurs wandering around the planet.
But, much to my immense surprise, documentation is still available at Avaya's web page. Here is the link.
http://support.avaya.com/japple/css/...leaseID=129574
You are probably interested in the Installation and Maintenance manual, that will tell you how to put it back together. Programming a System 25 is by command line, it does not use a GUI. And it is not intuitive. If you do not still have what the computer needs, there were several other kinds of terminals that would program the machine. All you need is a serial terminal.
Your best bet would be one of the companies that sell old (and I do mean old) terminal equipment that can access the PBX. What I used was an old terminal about the size of an old modem that fed paper through it, I needed that to keep track of what I did because there was no other way.
Good luck!
But more importantly, do you still have the login and password for it?