I'd avoid heat unless you intend to do a complete teardown. That might still be needed if the rust has gotten to the bearings. A SM 420 with square bearing balls sounds approximately the same as a rock crusher when it's turning. Fortunately, Novak sells a small parts kit that includes all the ball and needle bearings, but your bank card might need a little CPR!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!