Actually, it made no logical sense for Fabrizzio to stay in Sicily all those years until 1956. It's too small for the safety of anyone so notorious. Perhaps he had fled to the mainland of Italy, or to Tunisia (favorite haunts of Mob guys on the run) before coming to America. I'm just trying to find a rationale for the line, "...entered this country in 1956..."
In the novel, a Corleone gunman, not Michael, shoots Fabrizzio in his pizza parlor in Buffalo, NY. BTW: Puzo adds drama to the scene by having the gunman say, just before firing the fatal shot, "Fabrizzio, Michael Corleone sends you his regards." In real life, a lowly button never would have uttered that line because he'd never have been told that his mission was on behalf of Michael Corleone--if he had, he could have testified against Michael.