The phrase “part(s) of X” or “part(s) thereof ” is always preceded by some kind of determiner in the Book of Mormon text (namely, a /an, one, the, this, that, those, any, no, every, all, many, what, and whatsoever). The phrase “a part of X” is quite frequent, with 25 occurrences in the text (including here in Alma 9:34). But there are no examples like the one here in the early RLDS text (the 1874 and 1892 editions) for which part has no determiner (“nevertheless part of his words are written in this book”). Such usage without the determiner is common in English but totally missing from the Book of Mormon text. In accord with the reading in the printer’s manuscript, the 1908 RLDS edition restored the a that the 1874 typesetter had skipped.
Summary: Maintain in Alma 9:34 the original indefinite article a in the expression “a part of X”; there are no examples in the Book of Mormon of “part of X” without some determiner.