How can one be sure of the sincere repentance of an engaged man after he has committed zina, and what is the necessary period for that before completing the marriage, knowing that it is not possible to annul the engagement?

We congratulate the questioner on her sincere repentance. Part of perfecting repentance is to stay away from the young man until the marriage contract is concluded, because he is still a stranger to her. Repentance is regretting the sin committed and resolving not to return to it. Its place is the heart, and no one knows what is in hearts except Allah. So, if the young man says he has repented and goodness becomes apparent from him, he should be believed in that, and one should not investigate whether he is truthful or not. From the moment of his repentance, he is considered a repentant person, and there is no specific period that must be waited after repentance for the marriage contract to be valid. Marriage to him is permissible in this case. As for advice that would encourage him to repent and strengthen his faith, she can refer to answers no. 36139 and 35182.