Why not the KDX 200 / KX 100 hybrid? Those have been done before, and in fact I think it was Mrs. Vixen who has or had one . . . hello?
It's a great idea--the smooth power of the KDX, for which there are plenty of mods available, in the mid-size frame and suspension of the KX. Other hybrids are possible too, but the KDX motors are plentiful and cheap. Staying with one manufacturer keeps parts availability to a reasonable level and requires less custom fabricating.