I recently purchased a kit from TX Race in Spain to put 2017 to 2019 KX250F plastic on your 03-07 (or 08 overseas) KX, and needed a donor bike. I guess you can probably guess which bike I decided to update. Goliath got stripped down and I got started on the install.
The kit itself comes with a nice set of instructions (get a magnifying glass for the details), and an extensive group of brackets, bolts, washers, nuts, an airbox extender, and a seat (in addition to a set of plastic custom tailored to this kit). Some of the bracket pieces are plastic, while some of them are aluminum. They use rivnuts and the clamp on nuts in a couple of places, as well as studs where applicable. Overall the quality of the kit is very good.
I attached pictures of the instructions and all the parts laid out.