I've been playing with Paul Slocum's Cynthcart software for the C-64 :
The software is fun, but the interface leaves much to be desired. I would prefer a labeled control panel and a real piano keyboard. Of course, there are some complicating factors.
First, the switches on commercial music keyboards are matrixed completely differently than the C-64 keyboard.
See this diagram :
This requires a complete rewiring of the keyboard. I tried to rewire one of these by hand, and it was a disaster. Consumer grade circuit boards were not designed to be modified. They fall apart. PC traces come off the board, etc.
The plan now is to build a panel to hold 25 micro switches, and I will wire the switches by hand. If I wanted to build just one of these, I would hack it out, but this may be more universally useful, so I'm considering designing a panel that would be cut on a laser cutter, with holes for the switches. I can then wire the switches however I wish.