Contract of sale of a used car

Rohan, a car enthusiast, bought a second-hand Scorpio from his friend's friend, Aman, without a written contract. After a month, Rohan found out that the car had severe engine problems and was not worth the amount he paid. He immediately returned the car to Aman but Aman refuses to refund the money, saying it was an informal sale. Rohan has all the communication records (WhatsApp, phone calls, etc.) but no written contract. What would be the appropriate legal remedy in this situation?