This is a total shot in the dark, but one of her siblings, Valentin, is still alive and apparently still living in Budapest. He is 66 years old. I have no idea how one would be able to make contact with the son of the dictator, but if a physical address or email could be located, it may be worth trying to send a letter with the details of the car to see if he had any knowledge. Silly idea, I know. No doubt he is trying to live a quiet life, but at least you are not contacting him to ask about the regime, etc.