I don't know about the shock but the carb will definitely swap over, as should the forks.
The 2000 KDX220 forks are a vast improvement over the 91 units and the breaks are much improved. The KDX220 carb is a slightly different design that utilizes an Air Striker style jet block and slide. Its a 33mm carb vs. the 91's 35mm carb as well. The 91's carb will work great on the 220 just be sure to inspect the slide if the carb is well used to ensure its not worn.
I expect the 220's forks will slip right into the 91's triple clamps. If you were planning on using the parts on the 91 I'd certainly try and use the 220's top triple clamp. It uses rubber mounted bar clamps and positions the handle bars much further forward.
Dave, my dad has a 91 KDX which he is unable to ride for the rest of this season (injury). What kind of performance can I expect by swaping carbs? Is it difficult to jet, and will I have to go richer or leaner?
kawaJawki, I dont think the rear shocks are interchangable. The 91's has different linkage than the newer KDX's, so I suspect the shocks will be different as well.