03 audi tt mystery car
 

I picked up an 03 tt quattro from an insurance adjuster for $1400. The guy said he didn't know anything about the car. So I took the gamble took it home and found it I didn't have compression in two cylinders. Also the coolant flange was cracked so my assumption was coolant laid out it overheated blew a head gasket. So I took it apart check the head head is in great shape but found holes in the Piston. So looks like some kind of detonation what should I check and what would cause this? What would you replace or check to make sure this doesn't happen again once I replace the Pistons?