Doug,  Don't do it with some "automatic software" -- let me or someone
else of the arrangers' trade do it, manually, "the old way".  Or do it
yourself!  You'll learn much that way (using notation software is fine,
of course :-)).

I may be biased; see http://www.haraldmmueller.de/  But it's not only
the technical hurdle (which might have been solved by fancy application
of FFT or whatever).  But fitting some music to the tiny scale of a
20er organ is not something that can be done without knowing about
arranging.  In my maybe not vast, but also not too small experience,
writing down the melody and bass line is the least part of getting an
arrangement.

But the main point is that, in addition to the melody or bass, a 20er
arrangement needs "tweaking the melody" to that small scale; adding
trills, grace notes; and fine-tuning the note lengths (to give a very
rough overview of arranging for 20er).  All of this which will certainly
be missing from an arrangement for piano, big band, orchestra or --
"worst" of all -- a recording intended for accompanying a singer.  All
these are arranged in a totally different way.

Nowadays, even if I get a MIDI file of some music, I will _listen_
to it and manually write the melody, bass, etc., anew when arranging.
This is faster than fiddling around with the MIDI file to fit it to the
20er, and one gets to know the music much better for the other stages
of writing an arrangement.

So -- writing it down is the best way to go.

Best regards!
Harald M. Mueller
Grafing, Germany