In case you decide to build a small organ and you want to interface it to the MIDI world, for example to replace the paper rolls with real time MIDI files, but still cranking to feed with air the pipes with the bellows, I recommed to visit these two sites that explain how to do and sell the necessary subassemblies: the first one is from U.K, the well known J.V. Electronics company www.j-omega.co.uk, The second one, http://geocities.com/jdpetkov is a Bulgarian designer and has excellent references as a supplier of worldwide organ manufacturers. In this american site you can see an example of a practical DIY application :www.haraldmmueller.de/index.htm.
Also on www.alanpell.com there is a very interesting proposal to midify the crank organs. Here the MIDI board is particularly interesting because it also provides a MIDI encoder to read the crank speed. This feature allows to achieve a real time tempo variation proportional to the crank movement.
Naturally all above addresses do not exhaust the subject, there are in fact many other proposals even more sophisticated to Midify an organ , including the italian company SPRAE www.sprae.it, offering a broad range of encoders and decoders. I just outlined a few very simple and economic proposals well suited in my opinion to the world of buskers and band organs.