I agree with everything said so far about the proposal to build
a calliope scaled down to 20 or so notes, rather than the standard 43.
In particular, that such a scale would be best played mechanically
(e.g., by MIDI) rather than by hand.

One consideration not yet emphasized (though mentioned) is the question
of where the mechanical musical arrangements will come from.  If the
builder copies a standard "monkey" crank organ scale, such as the 20er,
26er (there are two 26er standards to choose from :-) or 31er, then
there is a vast library of arranged tunes out there -- though mostly on
paper rolls, not MIDI.  However, arrangers might be willing to prepare
MIDI versions of their existing work for a lower price than arranging
from scratch.

If the builder makes up his own scale, then he is strictly on his own
to make his own arrangements, or to hire someone else to make them.
And the arranger will have to get familiar with a new scale, rather
than work with one he's used to.

So, I strongly recommend a 20er, 26er, or 31er scale, suitably
transposed to the instrument's pipe sizes.  The 31er scale is actually
3-1/3 octaves, with many bass notes omitted, and is pretty capable.

By pitching the instrument higher, the pipes will all be smaller,
though the sound will not be as full and mellow.  That's another
tradeoff to think about -- smaller pipes, but more of them (more
notes).

Mike Knudsen