Private sales are less protected by the 2015 Consumer Rights so, to keep things as short as possible, the car legally only needs to be as you described it. It should be true to the advert and what you told the person who bought it from you. He won’t be entitled to anything just because the car is faulty or because the seller (you) failed to mention something in the advert. However, the law also takes fair wear and tear into account. So, for example, if the car had been clocked with a fake milage, the buyer would be entitled to a refund because you'd lied. However, if he knowingly bought a vehicle with 150,000 miles on it and then it broke down, he won't be entitled to anything. If the car isn't as you described it (I personally don't know what you said in the advert), then the buyer can ask for the difference in value between what you paid and what the car is really worth or a refund. My understanding is also that because the buyer has modified the vehicle (especially the engine), they aren't entitled to a refund, though.

Unfortunately, you haven't really given us enough details to help more - but these generals should help you make your own mind up as to whether he has any basis to claim money back from you. Based on what you've told us, I don't think he has rights against you. But my advice would be to seek free legal help from Citizens Advice.