How can one atone for the sin of reviling Allah and His Messenger, for someone who committed it in their youth, especially since the hudud punishments are not carried out in their country?

Expiation for sins is achieved through sincere repentance to Allah Almighty and increasing good deeds, for repentance demolishes what came before it, and good deeds efface evil deeds. Allah Almighty has promised those who committed major sins such as polytheism, murder, and fornication that their evil deeds will be transformed into good deeds if they repent, believe, and do righteous deeds, as stated in His Almighty saying: "And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever does that will meet a penalty. Punishment will be doubled for him on the Day of Resurrection, and he will abide therein humiliated – Except for those who repent, believe and do righteous work. For them Allah will change their evil deeds into good ones. And Allah is ever Forgiving and Merciful. And he who repents and does righteousness – he has truly turned to Allah with [acceptable] repentance."