i dont have major experence with kx500 (i am rebuilding 1 now) from what i can gather the the big problem is the con rods snaping, so defo renew it with some thing good, not wiesico (made in china now). kips mod on here looks easy and worth doing, pm kawrider hes got some speicial bolts. i am having my crank dynamicley balanced by criss applebee engineering, i think vibrations are the big killer with big bore 2 strokes so that should help, also using oem gudeon pin as it is lighter than a wiesico pin but still using the wiesico piston as it lighter that oem and with the window mod its lighter still. i am hoping that by reducing vibration, and mass from the piston end of the conrod it should reduce the load on the con rod. all the above is found from this site, just though i would save you some time and let you know what to look for
all the best
john