I recenlty found on eBay a "MUSIC 64" keyboard, with not many details, no warranty...I offered 10 euro to the seller and he agreed to sell it to me.
After some days I receined it, it was in pretty good shape and more imporant: it works!
I found some ROM images for its softwares (MONO64 and POLY64), here's a quick demo of the presets on the POLY64
You can edit various parametes of the synth enigne.