Looking at the picture I thing the question should be it is worth saving? There is a good chance that it would be more affordable and better looking to replace the cantle even if it can be saved. I would agree with others that this damage is most likely from what was used on the leather, most likely in an attempt to "help" the leather look better, and this kind of damage might not be worth the effort in the long run.

If I were you, I would take off the cantle and use it to make a pattern for a new cantle. There are may places where inexpensive leather pieces large enough can be found. The stitching is a saddle stitch, and I would advise using a high strength thread or leather sinew for the stitching. I would probably also replace the tassel tops on the sporran so that they look better with the new leather.