You can have a ton of fun with a KDX on a motocross track, but it is not a motocrosser, even with lots of mods. I used to race my 96 KDX 200 a lot, and did decent, usually between 10th and 20th with a full 40 rider gate. The big drawback is that you have to run in the 250 class. The track I usually race is huge, big straights, big hills, and deep sand. It's rather disapointing when I am absolutly pinned going up a large sand hill and a 4 stroker motors past me like I am standing still. My bike now is heavily modded, ported, worked head, pipe/silencer, bored carb, reeds, and I still get whipped on the start/straights (unless its muddy :) ). A bike that spits out around 35 hp can't really run with 45-50 hp beasts in the open. The suspension leaves a lot to be desired as well, heavy fork springs are a great help, but "real" MX bikes make my KDX feel like 1971 Eldorodo trying to run the Baja 1000, big, slow handling, and way too soft. What works good on the trails doesn't mean crap on a track.
So if you are doing it for fun, go for it. If you are racing with a competitive attitude, get a new bike!