We have a KX60 and an RM65.
Many of the parts are remarkably similar, for instance the clutch plates are the same, but there are many subtle differences as well. I wouldn't try mixing tops and bottoms. I can look at the frames to see if the entire motor might bolt up, but I know that the air box is entirely different.
I like to peruse the parts fiche at Bike Bandit to compare and contrast.
BTW - KX60 = RM60 any year
KX65 = RM65 subtle changes in some year models but if you match the year model they are identical.