Contrary to what is often claimed, the staff of the Metropolitan Museum of Art did not discover in 1967 the "Abraham" papyrus fragments that once had belonged to Joseph. They acquired them on purpose in 1918. This and other interesting facts can be read in New Light on the Joseph Smith Papyri by John Gee.
But were those fragments the source of Joseph Smith's translation? No. Gee makes some interesting observations about the fragments in the Met vs. what several eyewitnesses saw in the possession of Smith. Gee writes that:
Eyewitnesses from the Nauvoo period (1839-1844) describe "a quantity of records, written on papyrus, in Egyptian hieroglyphics," including (1) some papyri "preserved under glass," described as "a number of glazed slides, like picture frames, containing sheets of papyrus, with Egyptian inscriptions and hieroglyphics"; (2) "a long roll of manuscript" that contained the Book of Abraham;(3) "another roll";(4) and "two or three other small pieces of papyrus with astronomical calculations, epitaphs, &c."Only the mounted fragments ended up in the Metropolitan Museum of Art and thence were given back to the Church of Jesus Christ.The article's footnotes give the source documents for all of that information. Gee continues:
Those papyri went to Chicago and were burned in the Great Chicago Fire in 1871. Whatever we might imagine their contents to be is only conjecture. Both Mormon and non-Mormon eyewitnesses from the nineteenth century agree that it was a "roll of papyrus from which our prophet translated the Book of Abraham," meaning the "long roll of manuscript" and not one of the mounted fragments that eventually ended up in the Metropolitan Museum of Art.
One of the most common contention of anti-Mormon antagonists is that the Metropolitan Museum fragments are all the papyri that Smith ever had, and thus he translated from them, and thus he must be a fraud because he didn't translate what's clearly on those fragments.
The only problem with that argument is that the first part of it is clearly untrue.
Based thereon, it is much more likely than not that we don't have the papyrus that Joseph Smith translated from. Anyone who is concerned about the actual truth (rather than winning the argument) must admit this.
So, when it comes right down to it, it requires a fair amount of faith to believe either side of the issue.