Yep. You're definitely correct, that a roof skin is your best option. They're not being reproduced at this time, that I'm aware of, so a donor is your only option. The good news is that there are quite a few cars out there being parted out with good roofs. You can zip them off with a saws-all, after removing the windshield. The bad news is that separating the skin from the inner frame and fitting/welding to the car involves a lot of man hours. I'm not sure what all would be involved in replacing the entire roof structure as a unit (if it's even an option at all).
The c pillar roof skin overlaps, iirc, the quarter panel. There is a recessed seam there that was fille with lead. So it gets a little more complicated with that.
If the damage wasn't over such a large area, I would suggest repairing the existing metal... but that doesn't really seem feasible with the broad damage area.