We used to do a fair bit of photo restoration, both Sue and myself, although I think Sue has much more patience with it than I have. I think a fair objective is to repair an image just far enough and then stop before its basic character is lost. Here's an example of a restoration of a colour image, which as I recall was one of that Sue did.
It can take a lot of patience. This one was done using Photoshop with various cloning and other techniques. As said above, a most important part of the trick is of course knowing where to stop, before the image becomes an overcooked mess.