While the answer of there being no real repair for damaged Nickel is correct other than a re-finish, I HAVE been very successful with Nickel repairs this way.
While one can not "spot touch up" Nickel, what can be done is a very high polish of the damaged area. For instance...... I have a very early I/J frame Chief's Special (circa 1951) in Nickel and there is one particular screw in it that holds the Cylinder's Flat Latch onto the Bolt that I just can not find in Nickel, (yes, this one has a tiny screw - not the traditional nut-screw the latter models have). I was able to find a blued one and I took the bluing off and gave it a high polish so it looks just like Nickel. Because it is so highly polished and super smooth it never rusts either. (note: the original was too buggered up to repair).
So IF IT IS FEASIBLE, you can use a Dremel Moto-tool with a Cotton Flannel buffing wheel attachment and some Flitz to high polish the area to blend it in. Sometimes the area is difficult to get at with the tool and please make sure you do not do damage with the chuck of the Dremel. It really helps if you have the flexible pencil shaft attachment which is much smaller and more delicate for fine work.
This method has worked quite well for me.