Any KX number plate will fit on the KDX with a little bit of tweaking. I bought my bike with a # plate already installed and I noticed that he (previous owner) had to do a little bit of cutting of the plastic around some stuff.
You can purchase the fork guards at any hardware store ;). The while fork guards are made from built in vac tubing. They'll cost you about $2 to build. Just Cut them up the backside then make notches to correspond with the forks. Spread them apart to snap them into place.
The number plate is off a late model KX125/250 and needed about 30mins of work to fit in place just right. If you aren't worried about a perfect fit then it'll take you 30 seconds to fit.