There are a couple of good systems out there. Which one is to be
considered depends upon how original you want to keep the instrument.

If the organ is to have am authentic and "all original" restoration,
the Peterson system is good.  The digital player can be added to an
original relay system.  See:

    http://www.petersonemp.com 

If it is desired to replace the original relays and combination
actions, the Uniflex system is best.  Dick Wilcox, who invented the
system, is in charge of this company after the death of company owner
Tim Rickman last year.  See:

    http://www.uniflex.com 

I doubt that a regular MIDI system would support an instrument of this
size.  Please feel free to contact me with any questions.

Tom DeLay