I am really struggling to find someone else to play Michael and Vito (a good sign that the actor's performance could not be replicated), but here is the rest of my cast...

Tom Hagan---Dean Winters (Ryan OReilly on Oz)
Sonny---Micheal Imperioli (Chris on Sopranos)
Fredo---Danny Nucci or Lillo Brancato
Kay---Tea Leoni (I always thought Kay should have been someone better looking than Diane Keaton - no offense)