For a traditional calliope played with a keyboard, a 20 or 26-note
scale is very limiting.  The decreased number of pipes is achieved by
having a lot of keys on your keyboard which simply do not play.
If the organ (whether calliope, band organ, or reed organette) is
played mechanically, an arranger can change keys and use tricks which
make the missing notes a lot less critical.

An exchange which took place on MMD in 2002 gives a good outline of
problems with using a 20-note scale.

20er Music Scale & "Over The Waves" -- by Harald Mueller (021114 MMD)
    http://mmd.foxtail.com/Sounds/mueller1.html

20er Arrangement of "Over The Waves" -- by Christofer Noering (021115 MMD)
    http://mmd.foxtail.com/Sounds/noering1.html

20er Music Scale & "Over The Waves", Part 2 -- by Harald Mueller (021116 MMD)
    http://mmd.foxtail.com/Sounds/mueller2.html

A good arranger can still do a lot with a 20-note organ.  Listen to
this solution by the English contemporary master arranger Mel Wright:

20er Arrangement of "Over The Waves" by Melvyn Wright -- by Robbie
Rhodes (021121 MMD)
    http://mmd.foxtail.com/Sounds/mWright1.html

The "standard" Carl Frei 20-note scale (20er scale) used on many German
and English grind organs is described nicely on:

Small Organ Scales for the Noteur -- by Harald Mueller (050911 MMD)
    http://mmd.foxtail.com/Sounds/mueller3.html

If portability is a major consideration for your calliope, it is
certainly worth considering a 20er design without a keyboard, and using
the MIDI interface you mention.  There is a limited amount of free MIDI
music available in 20er format. A Dutch arranger sells collections of
his 20er pieces as MIDI files.  Perhaps you will also be interested in
developing your own arrangements, starting from the thousands of MIDI
player piano files available.

(I write as a happy owner of a 20 note home-built John Smith organ play-
ing mostly Mel Wright rolls.)

Wallace Venable