Here we have an interesting example of number shifting. First we start out with the plural “words of the prophet”. In the following relative clause, the text uses the singular verb was to refer to these words from a single prophet, Isaiah. The singular was can be considered either a grammatical error due to the proximity of the immediately preceding singular prophet or, more likely, as the typical occurrence of was in relative clauses modifying a plural noun (see, for instance, 1 Nephi 2:5: “in the borders which was nearer the Red Sea”). In any event, the use of the singular was here in 1 Nephi 19:24 seems to have triggered the use of the singular pronoun it later on in the passage (“and liken it unto yourselves”). From a semantic perspective, the words of a single prophet can be collectively considered as one text and can thus be referred to in the singular. For more discussion of such problems with number, see under subject-verb agreement in volume 3.
Joseph Smith, in his editing for the 1837 edition, changed the was to were and the it to them. The result is that the edited text in 1 Nephi 19:24 is consistent with a later example that also introduces a long scriptural quotation from Isaiah:
The critical text will, of course, follow the earliest textual sources in determining the reading for cases involving issues of grammaticality. Thus the singulars was and it will be restored in 1 Nephi 19:24.
Summary: Restore the singular was and it in 1 Nephi 19:24; the plural “words of the prophet” can be collectively considered singular in number.