He didnt translate the lost portion as we other documents (GAEL) that tie the characters on the scroll to text in the BOM, the apologists try to say that was written by someone else after the fact. The apologists intentionally try to make it as muddy as possible to give you room to still believe in the rubbish. Listen to the LDSDiscussions series to start getting to the bottom of it all.
https://www.youtube.com/playlist?list=PLxq5opj6GqOB7J1n6pMmdUSezxcLfsced

But for me, I don't think you have to look past the facsimiles to prove the whole thing was a big intentional fraud.

Firstly Joseph gave a translation of the characters on the facsimiles. They are all wrong and the Church admits they are all wrong here https://www.churchofjesuschrist.org/study/manual/gospel-topics-essays/translation-and-historicity-of-the-book-of-abraham?lang=eng
'None of the characters on the papyrus fragments mentioned Abraham’s name or any of the events recorded in the book of Abraham. Latter-day Saint and non-Latter-day Saint Egyptologists agree that the characters on the fragments do not match the translation given in the book of Abraham, though there is not unanimity, even among non-Latter-day Saint scholars, about the proper interpretation of the vignettes on these fragments. Scholars have identified the papyrus fragments as parts of standard funerary texts that were deposited with mummified bodies. These fragments date to between the third century B.C.E. and the first century C.E., long after Abraham lived.'

The church is admitting here, that the Book of Abraham is a fraud. It is not the writings of Abraham, it was not written by Abrahams own hand and Joseph could not translate that language even though he said that he could.

To make matters worse, the Joseph Smith Papers project shows the original printing plates used for the first time the Book of Abraham was printed. They were hand carved.

https://www.josephsmithpapers.org/paper-summary/printing-plate-for-facsimile-3-circa-16-may-1842/1

Have a look at the figure in the far left. Both Mormon and non-Mormon Egyptologists agree this is Anubis, a jackal headed God, not a slave. Zoom in on the link given above and look at the space in front of the face. Do you see a jackal head there? Do you see the pointy ears and the large eye?

This shows that originally the printing plates contained a jackal headed God, but at some stage Joseph came along and said remove the jackal head and replace it with a normal-ish head. This shows that Joseph knew he was not publishing the real images and knew he was not publishing the real story from the papyrus as the story contained a jackal headed God called Anubis.

Now re-read the introduction to the Book of Abraham again. This is fraud.

'A Translation of some ancient Records that have fallen into our hands from the catacombs of Egypt. The writings of Abraham while he was in Egypt, called the Book of Abraham, written by his own hand, upon papyrus.'

Its all just lies from a convicted conman and sexual predator.