I think you mean you stripped out the allen head so you can't remove the bolt. If so, you will have to drill out the head until it can break off, unscrew the threadded section then get another bolt to replace it.
If you stripped out the threads, you will need another base section. If you drill and tap that, it would be out of balance due to the larger steel bolt would be heavier than the aluminum it is replacing.
If you can get a dremel with a cut off wheel in there you might be able to cut a groove in the head and use a flat head screwdriver to spin it out. Hardware store should have a replacement one. If not, let me know and I'll drop one in the mail.