I've never heard of anyone taking those assemblies apart. They were just phosphated originally and should clean up nicely in EvapoRust. If they do have to be rephosphated, I don't think the tempurature is high enough to destroy plastic is it?
I doubt any replacements are available. You may have to find some generic metal bushings and drill out the bracket to the correct size to install them.(kind of like the door hinge pin kits in the HELP section at a parts store)