Q. May one make a siyum if certain parts were skipped (due to difficulty or other reason), but the vast majority of the tractate was learned?
Does one need to go back and do the few parts that were skipped?

A. Although there are different opinions on what is considered the bare minimum of having learned something, Orchois Rabbenu (2; Erev Rosh Hashana 9) maintains that even if it was learned without Rashi, if he basically understood what was being learned, he can do a siyum.
Askinu Seudoso (p. 209) mentions that the missing parts that were extracted by the censors, but are now available, don’t matter, but the rest does.
Horav Shlomo Miller’s Shlit’a opinion is similar and you cannot celebrate a siyum unless you actually finished learning what is normally considered as the complete tractate, at least in a basic form.
Poskim maintain that you don’t have to learn the tractate in the order it appears, later chapters could have been learned before the first ones, as long as in the end it was completed (Minchas Yitzchok 2: 93, Betzel Hachochma 2: 28)
It is doubtful if only learning the synthesis of a daf (kitzur hadaf), available today for people with time restrains or for chazara (repeating what was learned), would be enough for making a siyum.
The Rov maintains that he should nevertheless rejoice and make a seuda, even if it is not considered a seudas mitzva for the purpose of eating meat on the Nine Days or not fasting on Erev Pesach.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a