Holy shit, the motors already out..nice work!
Looking at the pictures, Id say it is a combination of detonation and clearances, it did get hot on the exhuast side and caused bad things to happen.
Scheides, definitely have the head looked at, Ron @ RS has a vacumn tool to determine the condition of the valve seats, id give him a call. You might be lucky and just clean up the rough spots in the valve pocket on the damaged one and be done.
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