ok here are a few possibilities:

Sonny - Dicaprio - dye job, jerri curl, best range to handle Caan
Affleck - dye job, jerri curl, diet, might be credible
Farrell - ""

Fredo - Ed Norton - think 25th hr. w/receeding hairline, perfect

Michael - Colin Farrell - but can he pull off one of Pacino's speeches

Don Corleone - Rober Deniro


Clemenza - Danny Devito
Harvet Keitel in a fat suit and makeup

Tessio - James Woods
John Malcovich