Interesting idea for James Gandolfini for Luca, he could probably pull it off. Some of the other suggestions are good as well but a lot of the people suggested i think are too old. Think about it, one of the most important parts of Sonny's youth would be the war when he was 19, so michael would only be early to mid teens and hagen would also be around 19. I think it would be quite cunning to play tessio and clemenza from part II as this would prevent having so many actors playing the same characters.

i'll make another post with my suggestions when i have thought about it more.