Is it permissible to supplicate for marriage to someone with whom one engaged in preludes to fornication, and then قطع (severed) the relationship with him after repentance?

Your marriage to this man is permissible if he repents and reforms himself. There is no harm in supplicating to marry him, for you have done well by repenting to Allah and cutting off your relationship with him. It is necessary to ensure his good conduct, religious commitment, and character, as the Prophet, peace and blessings be upon him, said: "If someone whose religious commitment and character you approve comes to propose to you, then marry him. If you do not do so, there will be tribulation on earth and widespread corruption." You may seek assistance from trustworthy individuals to inquire about him. It is permissible for a woman to offer herself to a righteous man within the bounds of Islamic etiquette. Before all that, seek help from Allah and humble yourself before Him.