I imagine the best people who have the knowledge and necessary equipment would be Canon. Perhaps you could find a 3rd party shop, but nobody I know of specializes in such a procedure.
Canon would probably do a "clean and adjust" on the lens, though I don't think they have all the parts anymore to repair it if something is wrong. I know they have to equipment to check for alignment. They could certainly take it apart and put it back together, but whether it would end up any sharper than it is now I don't know - and neither do they until they've done it. If it's not out of alignment now, rebuilding it won't make much difference.
I'd certainly talk to Canon first and see what they will offer to do. If they can't help then I'd maybe call someone like Precision Camera Repair (
www.precisioncamera.com) and see what they can do to help.