Help me identify this rotary encoder
The fact that Roland mounted this encoder on a small modular circuit board assembly does make it possible to think about finding an replacement encoder based mostly on the shaft mount, shaft size and knob attachment. The problem may be just a bit complicated if the existing unit also has a push switch action on the shaft. 

You could make a new circuit board to accommodate the mountings of the replacement encoder. It is likely that you could even find some encoders that can direct mount via the panel collar and then you could direct wire to the encoder terminals without a circuit board.