Here are the Polysex schematics. I marked additional parts for the spread control in red. Values are just suggestions, you will need to experiment a bit. The smaller the resistors, the bigger the maximum spread amount. The larger the capacitor, the better the filtering of the PMW signal.
With the values shown here the step response time is about 100ms, meaning the maximum modulation frequency when controlled via midi is in the range of 10-20Hz.