Sir John,
I've ridden the KDX at G.P.s and 24 HR events. They are remarkable; However, the materials used in the KX frames are/were superior. I don't know about the current KX frames('01 or later), I believe they're still a Chromo/steel mix.
The frames used in '99 have great multi-use geometry (not TOO quick on the front-end angles) and really good materials. I have friends in the off-road community that LOVE these frames.
You'll find that, until recently, these frames were used well beyond their model year.
I highly recm'd the '99 chassis ('99-00'? I'll check and get back...). As I recall, after '00 the frames became very SX oriented (geometry) and that may not be the best thing for faster paved courses...
Whatever you use: It would be a good idea to reweld/brace the frames. Under the loads of paved courses (Unreal traction and Superman braking) you can actually put stresses on these frames that they weren't designed for.
I was at Kawasaki last monday and checked out Chicken Maitasivichs Supermoto KX 500 and 250F. The 500 had a frame gussetted up just like my Desert racer. Infact, I'll try ro get a list together of parts and vendors from that bike. I think I'm gonna build one myself.
Cheers,
Manny