For others who consider this swap, the stock 2000-2002 KX airbox fits with the KDX motor and carb.
I have to strip and paint the bike so ignor the finish or lack of it at this point in the project.
I used 1999 KX250 forks and shock with 2001 KX125 swingarm and 2002 linkage on a 2001 frame. Apparently the linkage was revised in 02 to be better somehow.