Ok. Lost the top end of my ZR 500 to the PV learning experience. Found a good 800 motor and bought it w/ carbs, primary, mounts and bought a D&D pipe. Here is what I am guessing at needing to be replaced.

CDI ?????
Fuel Pump?????
Air Box.

Anything else that anyone can think of?

you need the tap the hols out for the mounts for the 800 and air box boot . the rear mount. and cut a new hole in the pan also you need ato drill a new hole for the pipe sapport and you need to buy the chain case bracket and the lower bracket

I have a 01' ZR800 CDI for $75.00 if you re interested, I made my 800 a 900 and went with a 03' 900 CDI.

Your Gearing will be different, so will your clutching, you will probably need at least 72 gram weights with the D&D pipe. I also have the stock ZR800 gearing and Chain (39/24?), $50.00 for that, $15 for the 72 gram clutch weights,What size Carbs come on the 500?? You may need those if they are different diameters, you will then need to upgrade them to ZR800 specs if they are compatible.What else........is the cooling system sufficaint on the 500 to handle a 800??