How can I atone for the sin of fleeing after my father's car hit a parked car, especially since many years have passed since the incident and it is difficult to reach the other party?

If you were an adult at the time of the accident and acted intentionally, or negligently, or were fundamentally unaware of how to drive, then you are sinful and must repent and seek forgiveness. In all cases, you must compensate the rightful owner for what you have damaged. The value should be determined by experts, and if that is not possible, then you should estimate the value yourself. If you cannot reach the car owners, pay the value to the poor and needy with the intention of giving charity on their behalf. If you find them later, give them the choice between accepting the charity or reclaiming their right.