If I were you, I'd probably just start with cleaning it really well, and running it as is.
Sure, the pitting will influence some air flow, but this ain't a scramjet motor, it's a carb on a woods bike. I doubt it will make a perceptible difference.
To be honest, a 96 KDX-200 has a slightly oversized carb to begin with, as evidenced by the fact that the KDX-220 (20 more cc's) with a smaller diameter carb will make more horsepower than a 200 (all other things being equal).
Just make sure the slide operates OK, and that the parts you can get at are as clean as possible.