All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The fornicator and the fornicatress are just like other sinners; if they sincerely turn to Allaah in repentance, their repentance purifies them. In fact, their sins will be replaced with good deeds, as Allaah says when mentioning the characteristics of the Servants of the Most Merciful (what means): {And those who do not invoke with Allaah another deity or kill the soul which Allaah has forbidden [to be killed], except by right, and do not commit fornication. And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated - Except for those who repent, believe and do righteous work. For them Allaah will replace their evil deeds with good. And ever is Allaah Forgiving and Merciful.} [Quran 68-70]
Also, Abu Hurayrah narrated that the Prophet said: "Whoever repents before the sun rises from the west, Allaah accepts his repentance." [Muslim]
Among the major signs that the repentance is accepted is that the situation of the person improves after it in such a way that the repentant person engages in acts of obedience and fears to commit sins and bad deeds. This is possible and it happens quite often, and there is no reason to think that it is unlikely.
As regards the prohibition of a fornicator marrying a fornicatress even after repentance, then this was reported from some of the Salaf (righteous predecessors) like Ibn Mas‘ood, ‘Aa'ishah and Al-Baraa' ibn ‘Aazib ; this was mentioned by Ibn Qudaamah in his book Al-Mughni.
For more benefit, please refer to Fataawa 208318, 84879, 82718 and 88697.
Allaah Knows best.