Changing a rib will depend on what ribs are available from the manufactures of barrel components.To have a rib made to an exact specification is vey costly.Buy worry not. A new bottom rib is no problem ,and I have succsesfuly extended top ribs on many occasions. There is the age old problem if the original has been engine turned as even if the same machine still existed the chances of getting a good match are very slim . But there are ways round this .