As i was saying before, doing this type of work takes a lot of time and dedication.
That one took me about 5 hours give or take. (and it still isn't really finished)
Make a pot of coffee and grab your GIMP
The best way to tackle an image like this is to look at the original and -
First grab the healing tool and go over the small imperfections
Next grab the clone tool for larger imperfections, and grab areas very close to the pixels you are going to cover.
Next see what can be duplicated and flipped. (the girls eye for instance)
I actually copied the left eye to another layer, and flipped it horizontally to get the other eye.
You should also pay attention to what features each person has, -- does the girl have her Dads lips? chin, cheekbones? Many of these things can be selected with the free select tool flipped and or rotated , and then re-colored to match with the curves, and level tools.
Many people do not realize when you open curves you can use the eyedropper in there to select a color on the image you want to re-color.Select it then just move the curve line to suit the color
(up is brighter and down is darker usually) - This type of touch-up works best on the layer with the selected part (before it's merged into another layer, like the original)
I should have kept the layer i merged so you could get an example of what i mean.
Oh well maybe next time.