If power was applied to the keyboard, it must have been related to whatever the original keying system was. Wouldn't need that for MIDI-only.
Have you checked which pair (s?) of contacts on each key close when you depress a key (with a DVM/ohmmeter of some kind)? All you need from the keyboard is a single identifiable contact closure per key, then wiring should be straightforward in whatever format you plan to use for your decoder. I can't make out the workings of the actual contact mechanism in the photos …
One of those two busses that run the length of the keyboard is (or could be used as) probably a common …. or cut in appropriate places into multiple commons for a matrix encoder, etc.