the 93 kx250 frame will work also with all engines with straight exhaust and front headstay... 88-91 have still the smaller swingarm shaft and motormounts. this must be bored to the right size. 88/89 motors also need the 90/91 cylinderhead with the front-mounting..
92-2002 fits without mods (except the smaller cases 94-2004), where you need bushings between engine and swingarm ..
for the 03/04 motors you need an 94-2002 cylinderhead for the frontmounts ...
...and any kx500 motors does also fit with light mods..