If you can replace the piston, do it. On the other hand, when I dropped an exhaust valve in the early hemi in my vintage dirt car, it did the same type of damage to one of my JE forged pistons, which were new less than two years before. When I contacted JE, they wanted to sell me another set of 8 pistons for another $1200. My builder magna-fluxed the piston and found no internal damage. He then had it heli-arced and re-magna-fluxed, and found no problems. I've run it in at least 30 heats since then successfully.
Bottom line : replace it if you can, but they sometimes can be fixed if the circumstances require it.
EDIT : I should add that even though the piston was repaired, the engine was torn down completely and thoroughly cleaned.