I don't know about the guy who played Agostino Coppola, but I'm sure Genco's voice is that of the actor who played him, Frank Sivero, because Sivero was a major supporting actor in "Goodfellas" and his voice, in English and Italian, is recognizable in GFII. As Tony, Alx and Noledge point out, it's not unusual to use the actors to overdub their parts during post production if the director or second unit chief isn't satisfied with how they sounded on the original soundtrack. I believe it's very unusual to use another person to overdub their voices, except if the original actor is playing a singer and can't really sing.
[Footnote: The best Don Corleone imitation I ever heard was done by Anthony Hopkins, who showed his stuff on A&E's "Biography" of Brando. It didn't surprise me that he was so good: When "Spartacus" was revived and remastered, they restored a scene that had been cut from the original theatrical release showing Laurence Olivier and Tony Curtis in a Roman bath. The Olivier soundtrack was messed up, so they got Hopkins to overdub his voice--perfectly!]