The original text of the Book of Mormon frequently uses the simple past-tense form of the verb for the past participle. Here we have “had begat” instead of the grammatically standard form “had begotten” (that is, standard for Early Modern English). The Book of Mormon text has two occurrences of the past participle begotten used with the perfect auxiliary have, although one of them is a quotation from Isaiah in the King James Bible:
Of course, begotten is used adjectivally when referring to the “Only Begotten” of God the Father (nine times). And the simple past-tense form begat occurs 41 times (all in the book of Ether). The critical text will, of course, retain the nonstandard usage “had begat” here in 1 Nephi 18:7. For further discussion, see past participle in volume 3.
Summary: Retain in 1 Nephi 18:7 the nonstandard past-participial phrase “had begat”, the reading of all the textual sources (including 𝓞).