enzo906 I assumed you were talking about Evo class with your question, but now realise you may have been talking about Pre 85. I think it is reasonable to keep that class as straight Pre 85, but I did notice a group in the US which also runs Pre 90 permits all KX500s, YZ490/WR500s and CR500s in the Pre 90 class because these bikes did not change at all apart from BNG.
Kevin