I'm not sure all will require oversize pistons, until they are measured and cleaned up. Trying to find a machine shop in my area that can do it. Not having much luck so far. Regarding cause, what about water in fuel? I had that problem early this summer. Engine stopped while motoring across the lake. Ended up being water in the tank.That sound like a good deal! Is it oversize pistons?
Maybe find a machine shop and have them look at it to determine what oversize you should be ordering.
Maybe have them chamfer the ports at the same time, I know I would do it when in there!
And like Jerry said, try and figure out why it clipped the rings. Chances are when you reassemble it you also fix any leaking gasket it had beforehand.
It could also be due to carbon build up and you just got unlucky.