Tony Stark is a boring turd in the comics but so entertaining in the IM movies. They could easily change up Stewarts character for the movies. You're not obliged to use all the lesser characteristics of comicbook characters in movies, just make the movie versions interesting and likeable if they're not in the comics. You can still maintain the core essence of the character.
Comicbook Tony and movie Tony are bothe driven by the same goals that define them only one is fun the other is boring, still essentially the same character. Same can be done with Stewart.
Heath Ledger wasn't the Joker from the comics but he was still very enjoyable. What I'm saying is you could easily use john Stewart and not have him be boring.