I dont know about the singer though. But i do believe some of the cr=haracters in GF1,Gf2 and GF3 based on real characters.

Hyman Roth potrayed Meyer Lansky as he's jewish and wanted to stay in Israel as a retired and returned Jew. In real life, Lansky tried to do the same thing.

Moe Green potrayed Bugsy Siegel. Eventho the cause of his hit wasnt the same as Bugsy but he was a Vegas bigshot who got hit by NY family.

Joey Zasa potrayed John Gotti due to his "exposure" to the outside world. No surpise in that.

However, it's quite hard to detemine who is DOn Vito real life potrayal.. i want to say Carlo Gambino due to people's love and power, but what about Costello? a man with honor, reasonable and have lots of judges and politicians in his pocket.

What is impossible for me to determine is Michael Corleone's real life potrayals? who could it be? Joe Bananas because he tried to whack some of the comission?