Here the 1920 LDS edition replaced the finite verb form had with the present participle having. However, there is some evidence that the original text allowed this kind of construction—namely, a present participial clause followed by a conjoined finite predicate. For discussion, see under Mosiah 23:1. The passage there originally read “now Alma having been warned of the Lord that the armies of king Noah would come upon them and had made it known to his people”.
Summary: Restore the finite verb form had in Mosiah 29:42 since such usage seems to be intended.