you probably won't find too many people looking to downgrade an 800 for a 500, you may want to consider just selling your used parts and putting that money towards a bigger engine. I know thats probably the engine in your sled right now and you probably don't want to sell it without having an engine already.
You may also want to consider the 600 and 700 engine, those two were FAST. you could keep up with the 800's. I'm not sure if they had an 800 twin in the gen II chassis though...
i believe the 98 wedge chassis and teh 99 gen II chassis had the same engines and exhaust pipes (don't forget you'll need more than just the engine... carbs, electronics, pipe... might want to just sell your sled and upgrade...
2005 Polaris Switchback 600: SlyDog PowderHound Ski's, SLP Air Box Mod, V-Force 3 Reeds, 144" track fully studded, Powermadd Pivot Riser and 4" block, Polaris Hand guards,