Will an apostate repay missed prayer or fast?
Whoever repented, Allah will grant him repentance. If a man commits any of the things that invalidate one’s Islam, then gained afterward the guidance of Allah, and he sought His Forgiveness, for such there’s no repayment. This is the consensus of Islamic scholars, because verily, Islam nullify what comes before it and repentance clear all what comes before it. Allah, the most exalted says:
‘ [O Prophet!] Tell the unbelievers that if they desist from evil, their past shall be forgiven’ Anfal verse 38
Also, the prophet, may the peace and blessings of Allah be with him said: Repentance clear off whatever comes before it and one’s acceptance of Islam clear off one’s past (missed acts of Ibadah).