In this passage we have two instances where the original text read “had began” instead of the standard “had begun”. As discussed under Jacob 2:12, such nonstandard occurrences of the simple past-tense form began as the past participle for the verb begin have all been removed from the standard text. Here in Mosiah 23:31, the change to begun was introduced into the LDS text in the third printing (in 1907) of the 1905 missionary edition; the RLDS text adopted this change in the 1953 edition. For further discussion, see under past participle in volume 3.
We also note here that the typesetter for the 1841 British edition accidentally omitted the had from the first occurrence of had began but not from the second occurrence, thus creating a very odd nonparallel conjoining of predicates: “and they began to possess the land of Amulon and had began to till the ground”. The following LDS edition (1849) restored the missing had.
Summary: Restore the nonstandard but original past participial form began in Mosiah 23:31 and elsewhere in the text; also maintain in this passage the parallel occurrences of had began in the conjoined predicates.