It would be much more practical to get a big bore kit designed specifically for a 800 zrt motor. You could get the cylinders and pistons and pipes as a kit from somebody like D&D or PSI Performance, or Hooper sells big bores. They all are extremely well engineered and make tremendous horsepower. On the other hand it is much cheaper to buy a sled on ebay that somebody has dumped $12,000 dollars into and is trying to get a bid of $4000 out of it. They are usually stacked up like cordwood for sale on there. Kind of like buying a muscle car. Let somebody else pour all the money into it and buy it when they become burned out on it. Good luck.
I agree and disagree with wat MIDMICHIGAN said. I would agree to buying a big bore kit. But the ebay thing, if there was 12k invested and its selling for 4k somethings up and I wouldnt trust ebay with past experience. I seldom use it.