Can't imagine who could replace Humph although Tim Brooke Taylor would probably be able to do a decent job. All other candidates that spring to mind are already with Humph (Willy Rushton, Alan Coren to name but a couple).
Good news about the Le Carre novels but I doubt that anyone would ever be able to replace Alec Guinness as Smiley in my mind. I can "hear" and "see" him even as I read the books. Anyone else will, like a Humph substitute, just not be quite right somehow.
My vote would have to be leave well alone and think of something completely new. You cannot replace the irreplaceable.