Nigel,  Your idea for MIDIizing a pump organ sounds interesting.  It
is not such a complicated project and many on the MMD site have done
similar things.  I have and I would be willing to share my knowledge
with you.

Take a look at my web page.  I have some experience with your new
idea.  Although I no longer make MIDI-controlled fairground organs or
glockenspiels, I still have many parts left over from the projects.
I can supply solenoids for your project which are very versatile since
they have return springs, length of travel adjustment, rubber bumpers
and yokes.  I ask $2.00 each, US dollars, plus shipping.  They are
24-VDC units with lots of power.  Email me for further information.

I can also supply the necessary wiring diagram and suggest suppliers
of MIDI-to-solenoid circuit boards.  Your project sounds like it is
a real possibility.

  http://home.talkcity.com/CookingCt/italiano34/FairOrgan.html
  http://home.talkcity.com/CookingCt/italiano34/GlockPage.html

Andy LaTorre
North Carolina, USA