How do you retouch a 100 year old photo in å curved glass?
So the photo itself is curved? Being so old, it would be hard to scan it without cracking the enulsion. You could try photographing it, and using the Spherize filter, or possibly a displacement map, to correct the curvature.
It woud be a good idea to use a long lens, and shoot from some distance away, to get the perspective of the shot as flat as possible.