I understand you better like to get indications, more than final solution.
Hence my suggestions:
1-separate the image into different areas (in the example shown below I separated skin areas from background and hair
2-use different tools for the different areas (in the example below I used gaussian blur 15 on the background, G'MIC smooth mean curvature on the skins, a detail increase on the hair)
3-at the end apply some colour improvement
Here an applied example (roughly performed, don't consider it an optimum result, you can get better outcomes with patience...)
Attachment:
2912_nataliajulietamara_DN.jpg
Thanks for the tips. I actually have my restored photos and I like the way they turned out.