How much of the text of the Gospel of John can be substantiated by early copies and quotes from early Church Fathers? It's interesting that a person will claim passages were added at a later date without considering the textual evidence.
A fragment of the Gospel of John dates in the first half of the second century.  We have complete copies of John in the fourth century codices.  My concern is answering arguments for parts of John being later additions before the fourth century.  An example of evidence, Irenaeus of Lyons, in the late second century, quotes parts of the prolegomena that some question it being part of the original Gospel.

But what John really does say is this: &quot;And the Word was made flesh, and dwelt among us; and we beheld His glory, the glory as of the only-begotten of the Father, full of grace and truth.&quot; Irenaeus - Against Heresies book 1
http://www.earlychristianwritings.com/text/irenaeus-book1.html


John, however, does himself put this matter beyond all controversy on our part, when he says, &quot;He was in this world, and the world was made by Him, and the world knew Him not. He came unto His own [things], and His own [people] received Him not.&quot; ... For when he had spoken of the Word of God as having been in the Father, he added, &quot;All things were made by Him, and without Him was not anything made.&quot; ... For that according to John relates His original, effectual, and glorious generation from the Father, thus declaring, &quot;In the beginning was the Word, and the Word was with God, and the Word was God.&quot; Irenaeus - Against Heresies book 3
http://www.earlychristianwritings.com/text/irenaeus-book3.html

I'm not concerned with John 7:53–8:11, which is not in the earliest manuscripts.  The rest of the Gospel is consistent in language and subject matter.  It does appear that John added chapter 21 as a postscript.  Has someone already compiled this information for the Gospel of John?
According to this data, 96.7% of the Gospel of John is quoted pre Nicea 325. It is the most complete quoted New Testament book and only less than 29 Verses are not quoted.
The document also has a section covering the Manuscripts. Manuscripts pre-dating Nicea covers 98.6% of John.
I've not verified the whole document myself (that would rightly mentioned take a lifetime career) but I find the Excel document extremely useful to find Patristic Quotes and its been very accurate.
Also, consider looking into early translations. The Peshitta is the earliest and this website has been a wonderful resource for me to find references. It seems to be the ongoing research of Dukhrana published online as they work on the various ancient manuscripts in Aramaic. I have not found anything near like this for the Old Latin.