Here the 1830 typesetter removed the so-called split infinitive by placing the not in front of “to have written”. It is unlikely that this change was an instance of conscious editing since all other examples of split infinitives have been retained throughout the history of the text:
Note that Alma 56:18 has a not splitting an infinitive and it has never been emended. The original split infinitive will be restored here in Moroni 1:1.
The idea that split infinitives are somehow wrong in English is a complete artificiality. For evidence of this, see the discussion under that subject in Merriam-Webster’s Dictionary of English Usage.
Summary: Restore the split infinitive in Moroni 1:1 (“to not have written”) since such constructions are found in the original (and current) text of the Book of Mormon; there is nothing ungrammatical about them in English.