What should I do after I scratched a car and did not inform its owner, and can I give charity equivalent to the estimated damage of 200 Riyals?

You are liable for the value of the car scratch as long as you are reasonably certain that you were the cause of it, because reasonable certainty takes the place of absolute certainty in Sharia. If you are unable to reach the car owner, you should donate that value on their behalf as charity.