Get some steel epoxy would be my method of fixing it if you can't remove the piece and replace it. Steel epoxy is basically moldable steel that hardens to the strength of metal. Spray some mineral spirits on the bolt, then a light misting of petroleum based spray (I would recommend going to a crafts store and picking up mold releaser from the plastic molding products). Then shape the steel epoxy around the bolt and to the remaining metal. It should cure fully in like 24 hours, to the strenght of metal. Remove the bolt just to make sure it can come out and go back in, then you should be good to go.

Something like this... http://www.stopit.com/fixstix.htm

I used to have some that I really liked, but forgot what brand it was. It wasn't the above product, but it was similiar. Just remember to do the misting of something oily on the bolt... anything from the aforementioned mold releaser to some sort of baby oil or something should do the trick. I learned the hard way that the metal epoxy WILL stick to the bolt, and can be difficult to convince to let it go.