I know of at least three systems for converting MIDI serial signals
from a computer to individual electrical drives for magnets or
solenoids to play the organ:

1.  John Wales' boards, from "jw Enterprises" in England.  These can
be had in 32 or 64 note versions, and cascaded for even more notes.
His circuit boards are very small and can be custom programmed to not
waste outputs on missing chromatic notes that your organ scale lacks.
I used one of his 32-note cards to drive my Raffin 31er, which would
have required 42 outputs of a purely chromatic board.  Web page is at
http://www.jwelectronics.free-online.co.uk/

2. Another similar card comes from Andre' Hut, Hut Music and
Electronics.  It's bigger, has 128 outputs, though you can buy it
with a smaller subset actually equipped.  Web page is at
http://www.gemhut.com/hmeproducts.htm

Both the above require separate solenoid valves (as from Peterson or
OSI) or Reuter type magnets, to "crack" the organ's tracker bar tubing
open to the air and trigger the chest valves.

3.  Spencer Chase and Gene Gerety are selling a system optimized for
player (and reproducing) pianos.  It combines both the valves and the
MIDI decoder card in compact blocks of 16 valves each.  It costs more
than the others (even when accounting for the valves not included in
the first 2), but is very small and easily spliced into the tracker bar
tubing, using its built-in nipples.  Nothing else to buy other than
power source.  Web site is http://www.spencerserolls.com/MidiValve.htm

All three of the above assume you have an organ playing paper rolls
or keyless books.  Keyed book frames would require Reuter magnets and
possibly a big changeover gang-valve, since keyframes do not just open
the tube to air, but forcibly switch each note's tube between pressure
and atmosphere.

I have successfully interfaced my Raffin 31er crank organ to MIDI using
John Wale card and Peterson/OSI valves, plus some careful woodworking
and nipple cutting.  It just _barely_ fits in the organ.  I'm writing
an article on it for the COAA Journal.

Please feel free to ask more questions.

Mike Knudsen