Sure it's the pistons? does it sound like a really loud valve tap? It coul be the water pump is bad now. Due to the bearings in the water pump, when the belt snapped it could have throwqn the pump off, causing the bearings in it to go bad.
This could also throw the ECU in safe mode and make the car run rich as fuck. This normally happens when the water pump and CTS are aligned or the CTS reads off the pump. It couldnt hurt to check it out, ive seen it before. Just put a screwdriver on the area near the pump and stick your ear to it...good luck.