This circuit was made by Peter Ullrich and it is based on the old P6Retro. He added hexadecimal switch for MIDI channel selection, MIDI thru output, and significantly improved the PCB layout.
 

Brief functional description:
    The retrofit circuit emulates key presses according the data it gets from MIDI and the state of the synth's keyboard matrix. It doesn't interfere with the analog circuitry of the synth and therefore is completely transparent: you can still play the synth even if MIDI data is received. Retrofit responds to the MIDI channel set by a hexadecimal switch.

Features (version 1.4):
    Receives MIDI notes. Hexadecimal switch for MIDI channel selection. Continuous controller number #120 or higher turns off all the voices (panic).

Zip file with schematics, layout and program code (.hex, v1.4) files

    When you located all that, prepare the PCB connectors: solder wires to one 2 pin connector, and make extension cable using both 8 pin connectors. Now solder the wires of 2 pin connector to GND and +5V.

    Now for a little mechanical engineering - you'll have to drill holes for DIN connectors, hex DIP switch, and mounting screws on the back side of the synth. Place the paper towels over the circuit to protect if from filings. When drilling use low RPM (you don't want filings all over the place) and start with small drill. If you don't have drill large enough you'll have to use round file to make hole big enough. When you are finished use some magnetized tool to collect all the fillings from paper towel, then remove the towel and examine the circuit again. Now screw the board to the back side.

    The last thing is to connect all the connectors to retrofit board. Connect power as following:
        GND -> SV15 pin 1
        +5V -> SV15 pin 2

    Then disconnect the 16 keyboard connector from the Polysix board and connect it to SV2 and SV1 on retrofit board, left-most pin of the keyboard connector to SV2 pin 8. Then connect extension cable to the connector where the keyboard connector was previously on one side, and to connectors SV4 and SV3 on retrofit board on the other side.