πιστιν is no different in meaning than πιστις [aside from the difference in case, accusative vs nominative]. It means faith or faithfulness. (See Perseus, Middle Liddell)
All the Greek text I quote below is from Robinson-Piermont's Byzantine Majority Text (2000) and the English translation, if not specified otherwise, is the ASV.
There is absolutely no way that Scofield is right in asserting that "The reference is not to personal faith, but to belief in the whole body of revealed truth." It doesn't match the context here, and it certainly cannot be said to always be true of the word πιστιν.
That does seem to be the meaning, or something like it, in Galatians 1:23
μονον δε ακουοντες ησαν οτι ο διωκων ημας ποτε νυν ευαγγελιζεται την πιστιν ην ποτε επορθει
but they only heard say, He that once persecuted us now preacheth the faith of which he once made havoc;
Here it means basically "the faith system," i.e. the Christian religion. However, notice that it has the article here.
But for a clear example that it doesn't always mean this (especially when the article is missing), See Matthew 8:10
ακουσας δε ο ιησους εθαυμασεν και ειπεν τοις ακολουθουσιν αμην λεγω υμιν ουδε εν τω ισραηλ τοσαυτην πιστιν ευρον
And when Jesus heard it, he marvelled, and said to them that followed, Verily I say unto you, I have not found so great faith, no, not in Israel.
Here, are we to suppose that the Centurion's faith that Jesus could heal by merely giving the command was "not personal faith, but belief in the whole body of revealed truth"? Certainly not! His faith was an analogy, that since he was a man "under authority" who derived authority from his superiors to boss the soldiers under him around, Jesus in like manner has such an authority from God to command disease to leave. There is no hint that the Centurion believed in the story of Sampson or of Gideon's fleece.
Now for Luke 18:8
λεγω υμιν οτι ποιησει την εκδικησιν αυτων εν ταχει πλην ο υιος του ανθρωπου ελθων αρα ευρησει την πιστιν επι της γης
It has the article, so we might suppose that it means "...Nevertheless when the Son of man comes, shall he find the Christian religion on the earth?" The side panel translation in the Emphatic Diaglott translates it that way:
"...But when the SON OF MAN comes, will he find this BELIEF on the LAND?" (Emphatic Diaglott, capitalization is in the translation itself)
That is, will he find this particular faith or some perversion of it? (They also clearly have interpreted the locational reference, επι της γης, to the land of Israel rather than the Earth, and it can mean either in Greek.)
I think there is a much better explanation, however. In Luke 18:8, the meaning of πιστιν is clearly "faithfulness" because of the implication that its possible it might not be there when the Son of Man returns. Compare it to such passages as Revelation 2:10 "Be thou faithful unto death, and I will give thee the crown of life." (Rev 2:10 doesn't use πιστιν [it uses πιστος], but it speaks of the same idea, being faithful to the end.) When the Son of Man returns, we he find any still faithful? or will everyone have given up due to persecution?
Romans 3:3 is an example of πιστιν meaning "faithfulness":
τι γαρ ει ηπιστησαν (lacked faith) τινες μη η απιστια (lack of faith) αυτων την πιστιν (faithfulness) του θεου καταργησει
For what if some were without faith? shall their want of faith make of none effect the faithfulness of God?
We find, then, that having the article doesn't always make it refer to the religion itself, but can still mean "faithfulness."
The CEB (Common English Bible) translates Luke 18:8 this way:
I tell you, he will give them justice quickly. But when the Human One[Or Son of Man] comes, will he find faithfulness on earth?”