Do you still have access to the original image so you can scan it again? If you do, try this.
Scan it, and then turn it 90° on the scanner bed and scan it again and then turn it 90° again and scan it again and then another 90°.
You'll end up with 4 scans.
Now open one of them - I usually start with the upright one. Now open another one, select it all and copy it into a new layer in the first image, set the blending mode to Overlay, and reduce the opacity so you can see the layer below and move it so you can match it up - you may need to rotate etc until you get it matched up.
Try with the different scans until you find good results.
I'm attaching three images. This picture is very badly cracked so I scanned it several times laying it in different directions on the scanner bed. The first two images are the two I combined to give the result in the third image.
Hope this helps, Margaret