They asked you the specs of the car it came from? If i were you id just tell them a load of crap and make up the spec of the car it came from. Tell them its a replacment engine taken from a corsa blablabla.
If they have asked they will probably use the donar car for the price of insurance. Maybe tell them a trained garage/vauxhall mechanic has fitted it due to the previous engine failing.
Its strange they asked about the origional car, most of the time you would get told 'we do not cover this type of modification'