I suggest that this is a bad idea. The appearance of Santa Claus is well established: he's a fat guy with a white beard in a red suit. If you make him a skinny, clean-shaven guy in a green suit, nobody will get that he's Santa Claus. If you try to get around that by having people call him "Santa", you will pull the audience out of the movie. Is there a reason (in-universe) why he wouldn't look like the stereotypical depiction? Nobody holds the rights to that, so you have no worries on that score. Come to that, is there a reason he has to be Santa Claus at all? If not, he could look like anything you want.