Check out what was done for Laurence Olivier in Sky Captain and the World of Tomorrow, on much less of a budget. Somehow, I think they'll get Brando right. However, I'd just as soon recast Brando as well, unless they want to limit their options in the probably sequel(s).
Oh, ok. I suppose they could dig up some other actor to play Zor-El, Jor-El's brother and have him be the disembodied voice/plot device. But I bet the unwashed masses who don't worship pre-Crisis Krypton would think that was totally stupid.