I watched a few of these KX500s sell over the last three years for around $4,000, and of course being raced/ridden by pros, most of them were pretty beat up. I think I was extremely fortunate to find a 2004 practice bike that was so clean, and I ended up getting it for $2,800! They just put brand new Excel rims on it and I also got the stock exhaust, desert tank, and extra spockets. Intersting thing about the front wheel is that it's 20 incehs. I guess they were experimenting with 20-inch front wheels for a while to see both how they handled, and to see if it reduced the possibility of getting flats. 20-inch tires are so hard to find now that I was going to replace the front wheel for a 21, until I found a source for 20-inch Dunlop D739s (which is what they used to race with on this bike) for $25/each.
Team Green was still racing (District 37 AMA Desert Racing) the KX500 in 2005, so other bikes and parts are still around, you just have to keep an eye out. When I found out that the 500 I bought just had the suspension and motor rebuilt, I couldn't refuse the deal. I have owned and raced a lot of bikes, but I never knew that suspension could be so plush! I think that's the best part of a factory bike because they put so much effort in this area. I wish I could find some of those Pro Circiut pull rods (suspension linkage) to try out, because mine doesn't have these. From what I've been told, the motors were mildly ported, and mostly stock.