Im not sure, but I'm pretty sure all this would void your warranty.
Opening the MS-20 mini is fairly simple:
Remove the 6 screws on the back
Take caution and remove the black coated metal clips holding in the beads
Gently remove the 2 cables from the jacks
Now you can remove the back completely and work inside
To install the PWM CV In jack:
Locate the test point labeled TP-13
The three pins above the test point are the potentiometer pins for the PWM knobs.
The pin directly above the test point is where you want to solder the wire for the PWM CV In.
Locate a ground via like the one pictured below. Solder a ground point to the via and solder a wire to it.
Solder the two wires to a jack as shown below
Now you have a Jack to send in CV signals to control your pulse width's modulation.
The Passive 4X Multiplier is simple:
Solder 4 jacks together. Same pins on each of the four. You can do as many or as little as you would want.
Many more to come!!