Is it permissible to marry a girl who has embraced Islam if she was previously a fornicator? And is it permissible to marry her without a guardian (wali) due to the absence of any of her family members?

If you are reasonably certain that the girl embraced Islam with conviction and is steadfast in her acts of obedience, then marrying her is a good deed and an act of devotion to Allah. If she does not have a Muslim guardian, a Muslim judge can act as her guardian in marriage, or she can delegate a trustworthy Muslim man to arrange her marriage. If marriage is facilitated, then it is a blessing. Otherwise, cut off your relationship with her, as she is a stranger to you, and seek another righteous woman for marriage.