If you're in New England you could sell it to me cheap.......
I had the same issue with my 04 Renegade 600 SDI. There are a couple possibilities. If your carb boots are dry rotted and cracked this will cause a lean condition and melt the piston. Your dealer should have checked that during the 2 rebuilds. If the boots have been checked/replaced then I will bet a gallon of semi-synthetic you have a partially clogged Injector.
I had the same issue. I bought the sled with a bad piston. I replaced the piston and ran 5 miles before I melted the first piston. I inspected the carb boots (again) and observed a little delamination but had no break through. I replaced the boots anyway and went back out. 15 miles later another piston.
It's a simple equation fuel and air. If the mixture is lean it's either too much air, or not enough fuel. I know the air portion was correct. That left the injectors as it is not carburated. I brought the injectors down to a local diesel injector repair/rebuild shop. They confirmed the spray pattern was messed up and was partially clogged. These injectors have a small filter at the inlet. That was what caused my problem. New injectors are approx. $150 - $200 if I remember right. I had mine cleaned for $25 each, including a new filter. Once I did that, I put over 2200 miles on without a hickup.
So there is my experience. Check your carb boots and have your injectors cleaned. These engines will go forever. I will clean my injectors every season. I liken it to cleaning the carbs before each season which I do on all my other sleds. This is the same process other than I can't do it myself.
Marco