Where it breaks would depend on the mast and how it is engineered and built, but realistically it will either deform due to increased compression or at the hounds.

The masts you have seen break are on the boats they were designed for. Would you put an A class mast on an F18?

Back to the original question:
Can you put a 18HT mast on an I20? Calculate the total righting moment ( boat weight x half beam + crew weight x ( beam + 3')) if they're within a reasonable tollerance ( say 15%) you're probably good to go but you've eaten into your safety factor.