Although “a marching” is not extant in 𝓞, there is room in the lacuna for the prepositional a. When Oliver Cowdery copied the text from 𝓞 into 𝓟, he initially omitted the a before marching; but virtually immediately he supralinearly inserted the a in 𝓟 (there is no change in the level of ink flow for the correcting a). Later, in his editing for the 1837 edition, Joseph Smith removed the a from the text. The critical text will restore the a here in Alma 57:31. As explained under 1 Nephi 8:28, the original text had quite a few instances of this archaic form of the progressive. For a complete discussion, see under prepositional a in volume 3.
Summary: Restore the prepositional a in Alma 57:31: “the armies of the Lamanites are a marching towards the city of Cumeni”.