Mo,
The old style head tubes (the piece that runs through the frame) has a smaller diameter and uses smaller bearings than the new ones. It is also shorter.
You have to change both tripple clamps and keep the old head tube. The upper clamp won't fit the old head tube because of the smaller diameter so you will have to do some work there. I had a mechanical work shop fix my upper clamp and it costed me ~ ?100.
Add to this the fact that your old brake caliper wont fit the new forks and the mounting brackets for the handle bar is different (not incompatible but different)...
Seriously consider getting hold of a basket case KX5 with the new front suspension and swap over the engine. It will probably be cheaper in the long run and you will have lot's of usable spares
//John