Is it permissible for a young man who repented of homosexual acts in his youth to become an Imam, and are his acts of charity and fasting accepted as a means of concealing this sin?

If you repent sincerely from a grave sin, Allah will accept your repentance. One who repents from a sin is like one who has no sin. Have a good opinion of Allah, and strive in supplicating to Him to conceal you and not expose you. Increase in performing good deeds that erase sins, and turn to the Quran by memorizing and learning it. There is no impediment for you to be an imam, and cast away these satanic whisperings.