Find Jeff Fredettes web site - excellent source of info on this topic. Under KDXposed, suspension info he indicates what shock spring rates for what weight of rider. You bike would have a 4.6 Kg spring if it is stock, if I recall correctly - and if you are a heavier rider, likely the spring rate is off. A new spring, depending on brand will cost from $85 (Eibach) to $110 (Race Tech). Newer KDX's (95 to current) are known to be somewhat softly sprung as well, yet actually have a heavier rate (5.0 Kg) due to swing arm leverage increase.
You can find used stock springs from a 95 to 04 fairly readily (do a post here on DRN classifieds). 95 to 04 springs will fit your bike. My son has a 99 KDX, but as he is light, needed a lighter weight spring. I found one from my add on DRN. I would offer to trade you my 5.0 for your 4.6, but I already found a 4.6 and I have decided to keep the 5.0 as the boy is only gonna get bigger!
If you simply go out and buy a new(er) stock shock and spring, you wont address the spring rate issue - but you can indeed find used shocks as well online.
So - do as cwweingartz suggested, then get a heavier spring under that scooter and ride on!.