All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
It is an obligation on you (Questioner) to compensate the owner of the car since it is a debt on you and Allah will hold you accountable for it, so you have to do your best to find him and ask for his forgiveness. However, if you couldn't, then you have to calculate the damages' cost and give it to charity on behalf of the owner so that Allah pays your debt to him in the Hereafter. And Allah Knows Best.