Akai S-20 Modification

I want to mod an Akai s-20 and put a potentiometer where the + and - buttons are to give the Akai s-20 a rotary knob for trimming like the Roland Sp-404 and other Sps. Anyone here good with electronics and could help point me in a direction on how to set this up?

post the schematics and i should be able to tell you.