Ok, WORK with me here. Obviously, it'd take some make-up on some (and a LOT on others):
Michael (try not to laugh!): Adam Sandler
Tom: Bill Paxton ("Morgan" from Tombstone)
Fredo: Freddy Prinze, Jr.
Tessio: The same one who played him in GF II (I swear that guy never ages!)
Brasi: Paul Sarvino
Clemenza: Dennis Franz
Young Sonny (try not to puke!): Tom Cruise
Connie: Leah Remini (now THERE'S some hooters!)
and Vito: DeNiro, DeNiro, DeNiro!!

"Vaya con Dios, Castle. Go with God."
"God's going to sit this one out." The Punisher (2004)