'71 AMX Roof

On the other hand, where is the roof rust, just above the rear window? Side window? Windshield?  The location of the rust and extent would be the deciding factor(s) on either the replacement or repair.  Also the experience and talent of the person performing the work is critical. If a body tech with rust experience can complete the repair(s) the cost may be cheaper than finding a roof replacement and the work to perform the repair. 

My recommendation to many folks is to pay a pro for the really difficult repairs and perform your own repairs otherwise. Almost anything is repairable with either time, money or practice. With that said, you can complete your own repair(s) with practice. If you mess something up, you can remove it and redo it again. 

I finished up a 66 Charger a few years ago in which the 70% of the metal around the rear window was rusted (up to 4 inches around the glass). The rear window sill (panel between the rear glass and trunk was completely rotted away with a improper installed prior metal patch that was riveted on and covered with bondo) along with the complete pinch-weld area that holds the rubber gasket for the rear glass was completely hand made and installed with several window trail fits to guarantee the repairs were correct.  The reason all the metal work was made from scratch on the 66 Charger, I could "not" locate a replacement panel anywhere for this customer. We searched the internet, forums and junk yards, but sometimes a car is rare enough that finding good parts, simply may not exist. So, you make the parts from scratch.  

Bottom line, almost anything can be repaired, the method is up to your parts availability, money, time and experience level of the person completing the repairs.