well i'm not for sure but i would think you should stick with the 600, as it is the small block, if your lookin for more power and speed why not get a big bore kit, i think that a 800 or 700 engine(big block) would take a lot of work to change in, and like you said, fabrication. Good luck, been having trouble with it or something?
ah well if you really had the time and effort i suppose you could fit in an 800, it would be quite a sleeper, but a lot of tuning and switching things around. I think it would be a good summer project if your in no rush. there are just so many things to look at if you do that. Cooling issues, clutching, gearing, etc etc. if you could find a zr 800 or mountain cat that has been wrecked but still has an engine, you could probly buy it and just piece it in there, just make sure you use all the parts off the 800, something might look the same but you'd rather be safe then sorry right? I thought you were wanting to put an engine in before the year was over.