the engines before ´92 have the shorter stroke.
by using the newer ignition (in case of right fitment of the statorplate in the also shorter ignition housing) ,you have to install it at the later timing mark. than follows maybe a little bit jetting and all will work properly...
older engines ´85-88 use the 8mm motor-bolts. ´85-91 the 15mm swingarm-bolt . in 89 came the 10mm motormounts and in 92 came the 17mm swingarm bolt . if you drill the engine-mounts to the right size , will all older kx250 engines fit to the newer frames. position of the lower engine mounts has never changed. its possible that you need some handmade brackets for the headstay ...
watch out that you also need the older swingarm of 92-93 , because in 94 and newer were the engine smaller on the rear and the swingarm became the bigger and longer bearings...
its makeable, but a little bit easier to bring a newer engine to an older frame.