How can I atone for a sin I committed twenty years ago, when I failed to perform the funeral prayer for a Muslim brother due to my condition at the time, and then saw the deceased in a dream angry, even though I am regretful and supplicate for him, give charity on his behalf, and treat his family kindly?

We understand that your sin was praying over a funeral while in a state of janabah (definition) (major ritual impurity), and we ask Allah to forgive you. You could have rectified the situation by performing ghusl (definition) (ritual bath), or by performing tayammum (definition) (dry ablution) if you feared missing the prayer, as Shaykh al-Islam Ibn Taymiyyah held. The expiation for what you did is seeking Allah's forgiveness and sincere repentance, and regret is repentance, along with the firm resolve not to return to it. As for washing the grave, it is not legislated and was not reported from the Prophet (peace be upon him) nor from his companions.