Dario, I think a variety of things would work. I might spin it in the lathe and polish it with some extremely fine emery cloth, but I think wet-and-dry paper of 250 to 400 grit rating would work quite well for the small amount of metal removal required here. Remember, the idea is not to remove, or even reduce, the scores, but to remove only the raised-up parts, so what is left will bear properly on the bore.
Remember to check the piston ring locating pins - do not let them stand out past the aluminium surrounding them.
