There is an Digital Audio Workstation (DAW) program called Reaper that
is becoming very popular. It can handle tracks with audio samples or
MIDI. It allows plug-ins to be written for it. The plug-ins can be
written in Python which provides a great deal of flexibility without
a huge learning curve if you are familiar with programming in any
language. You could write a plug-in to implement the key scale
including register stop settings if the instrument you want to model
needs this.

Although I haven't looked, there should already be plug-ins for
playing sound font files. It has a very reasonable pricing model. It
is free to use for 60 days and then you are on the honor system to pay
for a license (which is inexpensive). The program won't stop working.

There are several European café and dance hall organs that I would to
model someday. Are there any collaborators out there?

Jody Kravitz