Built it yourself?
Restored it yourself? (Acceptable, in spite of the non-cherished "10" plate).
These have a wooden chassis, don't they?
Dad, is that a real car? (Heard as I drove slowly past a primary school at going-home time).
We get these all the time but, as a senior contributor to the Lotus 7 Club's magazine told me when I had a Caterham, "answer their questions with a smile; be polite; gently educate them; and always remember that they are only day-dreaming - but you have got one".