oh mate
we are going back to about 2002 when we were playing with these,doesnt seem long ago but im 47 and fading fast,i think we had to as i said replace the whole front end and i think we had to use a smaller outer diameter stem bearing,also im pretty sure we had to cutdown the stem and retcut the thread on the top of the stem,we also put a longer swingarm on the rear with a kx85 rear shock and what we had was a fairly good handling bike with no grunt
lastly why do you want to do this because what i would do now instead of this is buy a crf150r which brings me to another frankenstein i built was a 1986 cr80 with an xl100 four stroke in it back in about 1992