dumb question, the original Nacra Inter 18 - is NOT F-18 compliant?

Could an Inter 18 still go boat for boat with the F18 fleet as long as it's not some top level event that requires measuring in?

Just wondering, since there are a few Inter 18s here with the carbon masts, and it would seem to be a lot of $$ to switch to aluminum mast just to be an "official" F18.

Any other differences between the Inter 18 and the F18 box rule?