Subject to correction, the "missing scenes" are only available in "full-screen" and not "wide-screen" formats; hence this fan edit was completed in "full-screen" format for consistency.
As Fanfiltration suggests, watch the original movies first: purchasing the Coppola restoration DVDs are an excellent way to watch them now.
Probably it was Coppola who started the progression towards adding back "deleted scenes" in rereleased versions of a movie. Usually one can realize why the director decided to delete the scenes in the first place. But for truly iconic movies, such as the Godfather series, it is fun to see what was left on the cutting room floor in context of the film itself and not just as "extras" on a DVD.
It was financial considerations that led Coppola to start this practice. NBC had already aired Godfather I and II separately, when Coppola was running into financial overages (for which he was committed to cover) on his Apocolypse Now epic.
So he sold NBC on the idea of running both films again as a four night mini-series, with the films re-edited chronologically. And he used the occasion to add about 75 minutes of deleted material, since for network television he had to remove some of the heavy violence footage.
If my memory serves me well, I believe that Coppola supervised this edit by satellite from the Philippenes (a first for the use of this technology), where he was producing Apocolypse Now. This version aired in November, 1977 as The Godfather Saga.
Given the rising importance of the home video market in the early 1980's, Paramount offered Coppola the opportunity to release this concept on home video. Coppola restored the missing violence, tighten up some of the editing choices and made some different choices on which deleted scenes were restored to the project. This was released in 1981 as The Godfather 1902 - 1959: The Complete Epic.
Following the release of The Godfather Part III, another compilation in chronological order with a different set of missing scenes was released in 1992 as The Godfather Trilogy: 1901–1980, but only on VHS and Laserdisc and never on DVD.
I believe that Fanfiltration has restored all of the missing scenes with the exception of those exclusive to The Godfather Saga version, since a high quality copy of that version is unavailabe (recent re-telecasts on television have been on cable channels that superimpose logos and other promotional material on screen during their telecast and it was never released to home video as this version).
There is a website that details all of the deleted scenes, but I do not have a link readily available. Some research on Google and you should find it (and be sure to share its address with us when you do).
Enjoy!!!