Here are some pics from inside the Y pipe.
Fantastic pictures of the y-pipe. So now you know why it failed, rebuild with good parts and this time run a wideband so that next time you'll see a problem before it happens. You won't need to be glued to the gauge either. Just enough to ensure the rebuild is being fed enough fuel and then the odd glance just like you would with the engine temp gauge. The difference is when you see DET on the gauge then you have the Wideband to tell you what's going on like a loss in fuel flow.
It's a lot better than being puzzled by the DET showing on the screen. If it isn't fuel flow then it's likely octane or perhaps way too much Ethanol, but the last two can be avoided for the most part so that leaves fuel flow. So if DET shows up and the Wideband clearly shows too lean, then it's time to stop the sled and look at the muffler temps. With a y-pipe like you showed I would imagine the muffler would have been red hot in places and at night it would glow in the snow like a neon light.
I've seen this on several occasions with a 600 SDI that comes to mind. It was scary hot with the plastic panel feeling like butter. In this case it was the injectors and a different driver. Sled was lent out to someone who didn't know of the particular lean spot at a particular rpm range and throttle position. The owner never rode long in that range so there was never a problem until he lent it out. Diamonddave makes excellent points from beginning to end. Everything he mentioned I learned from Doo and Poo SDI.
For the pistons, either stay with the same kit or go OEM. I don't see why after reading Dan's posts that you would contemplate the cheapo cast. Of course the break-in should be enabled after any rebuild, or do it manually which is more trouble.
By the way, the big thing that is missing on the older doo/poo SDIs is the DET warning. That is a gift on your sled to be used.
Right tools for the right job.