Pistons are pretty thick in the center and rarely ever fail there even after contact damage. In the automotive world I see it all the time when a timing belt or chain skips or breaks resulting in piston to valve contact. the head is pulled, valves replaced and the piston is left dented and does not fail but any damage 1/4" to the edge and its toast requiring replacement. I would worry more about the rod than the piston in these situations, at this point it was running just fine before removal I would ring it and not throw a piston at it unless the skirt, ring lands or top edge is damaged.
I had the timing chain let loose on a Pontiac 400 in a 69 GTO, it just acted like it ran out of gas but apparently the piston came up and hit a valve, bent the valve, broke the piston (the skirt, strangely) and hunks of the skirt (and/or harmonic balancer collar, I forget, long time ago) churned around in the pan hitting crank journals, scoring a cylinder wall, etc etc and made a mess. The hunks of metal I fished out of the pan were all smashed up and dented and rounded off...looked like they went through a wood chipper.
I pulled the head and sent it off to be fixed, put in a new bucket and rings, cleaned up the cylinder as best I could with hand tools, replaced the harmonic balancer and buttoned it up. Ran it real easy for the first 1000 miles, about, and I thought I was good to go. But then it developed a knock and then it threw a rod, dumping oil all over the road. That was the end of that motor!
