I have recently bought a KX 500 from Price Racing out of Las Vegas, It was one of Shane Esposito's practice/race bikes. The bike is awesome.
Some of the things they did to bullet proof it for desert racing were to:
Heliarc weld the entire frame.
Reweld the radiators.
Weld on protective loops for the waterpump and ignition.
Install a Hinson Clutch.
Take apart the airbox and seal it with good sealant
PC Valving for Desert 210lb rider Pro Level
PC KYB works fork kit
Installed a Pro Circuit Bladder in the shock
IMS footpegs and mounts
Renthal Bars and Sprokets
Mild Porting by PC
PC Pipe and jetting specs.
Extended clutch arm
Balanced crank from Falicon
Rear Disc Shark Fin
Excel Wheels, Talon Hubs, and Bulldog Spokes and Nipples
GPR Anti Vibe Bar Inserts/Stabilizer
BRP Chain Guide
IMS Tank with dry break valve
The only thing I didn't get was the trick PC forks that Team Green' Desert racers are using on their KX 500s. Great pic in new Cycle News shows forks that have much less underhang. I am sure they are probably late model KX 250 forks with nitiride coating
Since I have had it I have drilled out the gas tank mounts and ran a 6mm or 1/4'' bolt through the tabs to tie everything together. Now it is grade 12.1/8 with a double sheer.
I replaced the crank bearings with close complement bearing that have two extra rollers as compared to stock. Kaman Bearing in San Diego had them.
Installed time serts in all of the head studs.
I also drilled out the top shock mount on the frame and replaced it with a longer bolt that enabled me to get al nyock nut on it. The bolt I used was off a 1987 Fourtrax 250. It is the upper/lower A-frame bolt. They come in different lengths so make sure that it is long enough that the spherical part of the shock won't be riding on the threads.
Keep suspension and motor bolts tight.
This bike hauls and rides super smooth. I am getting ready to start racing again after a long layoff, that whole school thing.
BE SURE TO USE LOTS OF LOCTITE, BOTH RED AND BLUE!!!!!!!!!!!