How is the chuck attached- - - -with a thread, or a Jacobs taper? I'll second the motion to replace the chuck, but getting a Jacobs taper off the spindle is a real challenge. There's a pair of slotted wedges that are used to remove that type of chuck, along with a big C clamp or a hammer and a backup weight. Unscrewing a threaded chuck is a lot simpler. Victor Machinery Co. (Victornet.com) has a good assortment of chucks at pretty reasonable prices.

www.ebay.com/itm/157537628402?

You could also grind that "too big" key shank down very carefully until it fits the hole in the chuck.
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!