Here the 1840 edition replaced the simple past-tense began with begun, the past participial form in the standard language. This change seems to have been accidental since the began in the following main clause was left unchanged (“they began to contend with me”). We have one example of this dialectal intrusion in the manuscripts, but only momentarily:
In this instance, Oliver Cowdery initially miscopied began as begun in 𝓟, but then virtually immediately he corrected the u to an a. There are no other instances of this particular mix-up in the text. More common is the intentional use in the original text of the simple past-tense form began as the past participial form, in place of the standard begun. For discussion, see under Jacob 2:12 as well as under past participle in volume 3.
Summary: Maintain began rather than begun as the simple past-tense form in Alma 9:1 and elsewhere in the text.