Regarding a modern CPU to replace the 8048, I replaced my Polysix's Key Assigner with an Arduino Mega. Here's a quick overview...
I've shared my code for the Key Assigner, though there are some caveats: (1) it's super ugly code, and (2) it assumes that you've already replaced your keybed with a MIDI keybed. With that said, the code is here:
Chip