I think AMinkman is right. You might have to get the suspension revalved/resprung to compensate for the larger wheels & longer swingarm.
Go to the suspension forum and ask for Jeremy's opinion on this.
Why are you looking to do the big wheel set up? If it is to race in the AMA school boy class the KX80 meets the requirements. You are allowed up to a 16in rear, 19in front and to 100cc, but you are only required to have at least 48in wheelbase, and minimum 14in rear. The CR80 expert,and the KX100 cannot race in the lower 80 class, but all the MX80s can race in school boy class.