Nik,
The poster's question was to restore or not to restore a barn find. And so I keep my answer the same.
I agree absolutely that a car that has retained period changes that were of historical or racing significance increases in value. If that car actually had a race history of note. And these changes should be retained if the owner decides to restore the car. That said Mrs. Darling, (although an incredible woman and character) did not have a car that fits your criteria. It was not modified so she could race it and she - at least to what I have read - never won a significant race with the car.
But I agree that a car does not have to be pure to be valuable.
I agree because of the existence of a car like this:
http://www.oldyeller2.com