My quick response (I'm removing myself from responsibility for being
intelligent and fair): "If it ain't broke..." is an idiom and therefore
should not be altered. I would rather consult Webster's DICTIONARY OF
ENGLISH USAGE than Fowler. Unlike Fowler, who merely guesses,
Webster's tells the real story behind"ain't" (it's a low economic
class pronounciation of the same root word that upper class citizens
pronounced "aren't"; out of those two different pronunciations grew
two different spellings, which are now considered two different words).
Aren't, in other words, does not dervive from "are not."
        --Bill Murdick