Your image is greyscale. That's good. It makes things easier as you only have one channel.

I suppose PSP also has levels, curves and layers with layer-masks?

Well, first you should make the photographs like it should look like, regardless of the problem area's. Best use curves for this. It's easier to do this from the start, and then begin the rest.

When this is done, and saved, copy it, add a layer with a layer mask and load the copied image as the mask. Because you have a greyscale already, it loads easily as a mask.
Now in the mask, paint everything that is unaffected with black, leaving a safe border around the affected area's.
With levels, force what is left of the mask to high contrast.
Activate your layer itself and drag with curves untill the area's match (or are very close.
Do the details by hand with cloning or a similar tool.

Five or six are fun to do. Five hundred would be a bore.

There are no Borg. All resistance is fertile.