canyncarvr is right. Look it over real good. Every thing from the triple clamp stem fit to the stem bearings, and the steering stop. I have done this conversion on 1996 KDX and it is an awesome conversion with great benefits if it is done right.
I'm a believer in the condition of the whole bike. If the rest of the bike is well taken care of and other aspects of the bike look to have been done with care and good maintence, then most likely the conversion was done with the same care.
as for the set up of the KDX with KX forks. It's a great mod., but just like any suspension you will have to dial it in to your weight, riding level, and conditions.
good luck