Only USDM model conversions and part interchanges (other than specifically allowed) are legal, so the Silvia conversion would not be SM legal.
For fenders, any material can be used for the front fenders, and both bumpers (but not the bumper support). The rear fenders can only be cut up to the vertical plane of the hub face, and only for tire clearance. This allows you to make the cuts and modifications generally required for over fenders or flares.