I seen this replica on a Australian forum about 7 months ago. He wasn't sure of the brand but he only wanted a couple hundred for it.
Would you base your FRP Replica off an OEM Bumper? I really don't think the work would be worth it. Shipping would be kinda high. How would you make the grilles? No grilles doesn't really make the aero bumper look good.
There is a few companies make FRP Replica Silvia rear valances now. As for sides OEM side skirts aren't really to expensive and they are indestructible.