Can you transpose down and up (I've got an Alpha Juno 2 but can't remem if you can or not?) as that would get round it another way..
I reckon Roland would be expensive, I'd e-mail kenton electronics in London and see what they'll charge you, they did a midi retrofit and service to some 303's I've had and sold (and kept) and were always spot on.
tech@kentonuk.com