I used the 89-04 crank shaft main bearings and rod kit with zero issues in my 87 KX500. I just rebuilt my old crank with newer parts. I also used the 89-04 piston/ring set in my 87 500. The 87 piston is only one ring where the newer ones where two rings.
i think he i talking about a complete swap
what i wrong with the orginal crankshaft?
the primary gear is going to be different fit , keyway /nut 86/7 ,splined /cirlip 88on
the clutch looks like it has different gear [ in the Clymer manual]83/86 and 87on so the later primary gear will not match
it may fit the 87 if it has the later clutch gear but not 100%on this