I have no idea, but I would really like to see a good movie about this so I hope somebody chimes in. Wikipedia says that "Last Days of Mussolini" had Rod Steiger playing Mussolini and Henry Fonda was in it too. They're both pretty good.

Playing Mussolini would seem like a kind of low-upside choice for an actor. His persona was so, like, self-consciously absurd that it seems really easy for somebody to look ridiculous playing him.