If I had a time machine,I would send Jaime-Lynn Sigler (Meadow Soprano)back to 1990 and given her the role of Mary.

From Wiki:
Julia Roberts was originally cast as Mary, but dropped out due to scheduling conflicts. Madonna wanted to play the role, but Coppola felt she was too old for the part. Rebecca Schaeffer was set to audition, but she was murdered. Winona Ryder dropped out of the film at the last minute.
(My personal opinion,given the above choices, would be that Winona was the best pick).

Also, Sofia was in all 3 Godfather films. She played a 9 year old in the Ellis Island scenes in part II.