The pistons wear and the rings wear and the cylinders wear. Usually if done in time the first rebuild can be rings only. But if left too long or it's the second rebuild you need pistons and a boring, oversize. Now you need one piston. I don't like the idea of one piston. You should match it to the other piston. Things should stay balanced and matched. The smaller piston is a very poor match for the one you already have. And you prolly have enough cylinder wear that the original piston is loose in the bore? So try to find two bigger pistons than the one you have in fact it might be a good time to buy oversized pistons and get the thing bored.
I had second oversized pistons in my R90/6 but they were worn out. I was going to put rings on them but then another set of second oversized, not worn out, pistons showed up and they fit my already bored cylinders perfectly. talk about luck.