My example for a low-cost, low-tool-need "MIDIfication" is shown at
http://www.haraldmmueller.de/ , "MIDIfying a 20er organ".  I built a
separate gadget with standard electro-mechanical organ valves that is
set on top of the tracker bar.  For all the parts (including WWW links
to sources and approximate prices), see the web site.

I plan to build another similar "MIDIfication unit" for my Hupfeld
Phonola player piano (73 playing notes), also without changing
_anything_ in the piano, just a valve-cluttered board that extends
on the front of the piano from the tracker bar.  But first I have
(urgently) to rebuild its pneumatics, otherwise there's not much use
for the MIDI input, huh!

Professional systems sometimes use not organ valves but parts from
pneumatics systems (e.g. Festo: www.festo.de).  These are much more
expensive, as far as I know, but much more rugged, mainly because they
are inside a closed housing, and because they are built for much higher
working pressures than usual in musical instruments.

One important item: Tempo control.  For instruments playing with
"constant speed" (player pianos, motor-driven instruments), this is
simple: the computer will control the tempo.  However, for manually
driven [hand-cranked] organs, it is necessary to provide electronics
and software so that the output tempo depends on the crank's rotations
per second.  This is actually much harder than the MIDI output.

Regards

Harald M. Mueller
Bavaria, Germany
http://www.haraldmmueller.de/