There are certainly some very real people in all of the GFs. I guess the corleons themselfs are a lot of people whom Puzo couldn't ID to easily plus the GODFATHER is a work of fiction after all! I would not like it to be no other way.
However let see what little me could get:

I was told that Johnny Fortain was Frank Sinara
Moe Green is Bagsi Segal
Hyman Roth is Meir Lenski
The Corleons business in Havana were probebly true...