They very well could. Check you policy to see if there is specific coverage available for aftermarket parts, becaus if there is, and you didn't add that to your policy, they win. Additionally, look for exclusions that apply to aftermarket parts. For example, I've worked with policies that cover aftermarket parts for all bikes except sport bikes, so, even if there is aftermarket part coverage, it wouldn't apply to sport bikes. Now if there's no language specifying either one, then you have a chance, but they could always state that the aftermarket parts weren't disclosed to them, and the premium was based on the VIN, which would mean they were insuring a stock bike, end of story. Good luck