Scratched cylinder bore

I've had a couple of these over the years. It's common for the plain lock rings to wrok out or break under hard abuse, then the pin moves around and gets the bore. That one piston will bneed replacement - I wouldn't trust just sticking a new lock ring in that groove. So you're looking at a sleeve and pro honing plus one piston of the same type as you have. That's not considering taper in the bores. With signs of abuse as you note - the ring lands are probably worn too. So if it were me, I'd just expect to rebuild the lower end properly, stick in some oversize pistons (or a performance improvement set of hypers) and go that route. For the long run, you know it's hurt - it needs work to last.