[olddog]

From what I could see in the pictures, it looks to me like the piston land broke and the pieces that came off the piston did all the damage. I'm not at all certain, but I do not think the valve hit the piston. Rather the chunks off of the piston hit the valve.

I have seen pistons break like this when the cylinder leaned out. The lean condition overheats the piston. The piston expands and become larger than the bore. This sticks the piston to the bore, and then the piston land breaks off.

Look on the cylinder walls and see if there is aluminum rubbed off or hard scuff marks or both. When the piston is out, look closely at the lands on the sides of the piston.