Martin Harris lost 116 pages of the Book of Mormon translation in 1828 however the content was largely not lost because Nephi has already made a record of the same time period as instructed in 1 Nephi 19:1-2:
And it came to pass that the Lord commanded me, wherefore I did make plates of ore that I might engraven upon them the record of my people. And upon the plates which I made I did engraven the record of my father, and also our journeyings in the wilderness, and the prophecies of my father; and also many of mine own prophecies have I engraven upon them.
And I knew not at the time when I made them that I should be commanded of the Lord to make these plates; wherefore, the record of my father, and the genealogy of his fathers, and the more part of all our proceedings in the wilderness are engraven upon those first plates of which I have spoken; wherefore, the things which transpired before I made these plates are, of a truth, more particularly made mention upon the first plates.
Mormon goes on to explain in Words of Mormon 1:7 that he included Nephi’s writings for an unknown reason:
And I do this for a wise purpose; for thus it whispereth me, according to the workings of the Spirit of the Lord which is in me. And now, I do not know all things; but the Lord knoweth all things which are to come; wherefore, he worketh in me to do according to his will.
Would Joseph Smith really have gone to this trouble to make the Book of Mormon look convincing?