How have they found out?
If they have proof of you driving the car like that, then not much you can say really, other than try the excuse that you didn't think it mattered as the car has alloys anyway and the bonnet doesn't make any difference.
I know at a local cruise the police used to take photos, some off which were apparantly sent of to insurance companies to check everything on the car was as declared.... if thats the case then just tell them that the bonnet and wheels are for off-road use only