Another cool retrofit worth mentionning is our own Kevin Lightner’s Opto Key system. For those who don’t know it yet, this is a drop in replacement optical key contact system for Minimoog D, with no holes to drill or any modification to perform, other than removing the mechanical contacts and installing the small boards.
It hasn’t got very extensive MIDI capabilities (in and out and simply note on/off), since it wasn’t designed as a MIDI device, and was a last minute add on feature. But it can be used also as a MIDI to CV source for pitch and gate signals (although again it wasn’t designed for such use, but it’s there nonetheless).
Just thought I’d mention it, since I’m thoroughly satisfied with it. 
