Is there any reason Roy needs to be a stacked like a bodybuilder? I thought part of his thing was that he had unusually average physical attributes for a fighter.
For the purposes of casting I think the manner is much more important than build, provided the actor's in reasonable shape. Roy's a bit of a challenge because while a comedic character he's also prematurely world-weary and a straight man for most purposes. I'm not sure any of Samuel L. Jackson, Terry Crews or the Rock fit the part, age notwithstanding.
Andre Braugher is too old, but I see him playing Roy rather more convincingly. Among more realistically aged actors who've been mentioned already, I think John Boyega is a decent call.
It's hard to picture live-action individuals playing the characters though for the same reason that Rich decided against an art upgrade for the Julio Scoundrél comic: when you've got used to the characters as stick figures, nothing else will look right.