We had a hot rod let go on a kx 85 some years ago , broke at the big end side , lucky it happened at low revs and only cracked a case , and we could repair it , I stick with oem usually. Another thing I do to kx85 is to drill the holes that feed the main bearings 1mm larger as I had a problem there once ,seem to help it last longer , but I would look at the tuning side of things close, compression ,jetting , the casting behind the stator were the crank comes out ,were it dips in , they had trouble there to , is the crank cases sealed nice and tight.Hope you find out what it is , and don't give up on it , the cure is out there. Try a stock head , see if that helps?