Is it broken? I really can't fathom why such an organ would even really need MIDI. 
This console might already have a form of MIDI, you might want to check for that. During the ADC era.
If you want to go MIDI, that's fine, but I should probably point out that you will likely, unless you are real good with electronics, lose the function of your capture action, all your pistons, and all those beautiful white moving drawknobs, in addition to any of the other functions that make this console such an exception instrument. You see, unless you have an intimate knowledge electronics and can make most of this stuff from scratch, you are going to have a very hard time getting all the stuff to send a MIDI signal in a cost effective manner. It make no sense to me that someone would have a beautiful three manual drawknob console with the works, but be willing use a mouse to make registration changes! [+o(]
I you are going to do it, do it right I say. Here are a few places that can help you with that:
I highly recommend that, if you don't think this is something you want do completely by yourself, you find a good tech who knows his Allens. You should insist that the organ maintains full functionality in addition to having MIDI. If you can't do it yourself, then I think it safe to say that with each additional function you have midified, the increases exponentially. It would probably be more effective, (and cheap), in the long run to find a much smaller, simpler organ to do that with, as you lose fewer functions this way.