About 6 months ago I bought a MIDI conversion cartridge for my
37er Hofbauer crank organ from Mr. Watterott of Thueringer
Musikantenschmiede, http://www.ziehorgel.de/ 

I could not be more pleased -- the MIDI unit works very well.  It
came with lots of music.  My friend sent me a lot more and I have
been able to download and rework several songs from the Internet
that work well also.  In addition, I can plug my MIDI keyboard into
the unit and play it manually.  This aids in checking the tuning.
This little box opens up a lot of options for this instrument.

Thaine Morris