Is it permissible to supplicate for marriage to someone with whom one has committed fornication, after sincere repentance and abandonment of the act, along with a prayer for their guidance?

Praise be to Allah that you have repented from what you excessively indulged in. Supplicating to Allah the Almighty for marriage to a specific person is not rejected or forbidden by the Sharia. It is better to supplicate to Allah the Almighty to choose for you what is good, for Allah's choice for His servant is better than the servant's choice for himself. Allah the Almighty says: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not." (Al-Baqarah: 216).