There wouldn't be too much to add to the body if you use the
GT3 from Tamiya as the starting point (that is the car the RUF started with). You would have to tweak the front clip a little, mainly in the intakes and the front splitter/spoiler. It looks like the fender flares would need to be scratched, but that would be easy, only some sheet styrene, no real body work. The mirrors could be made from the kit parts, they look a little smaller on the RUF. The spoiler can be found in some of the Japanese "Detail Up" add on kits, made by Aoshima, Fujimi, and possibly Tamiya. The changes that RUF made to the car are mostly in the innards, lighter engine, better suspension, more power, etc. They really only added a body kit and wheels to the outside. I'm not too sure about the interior, although it shouldn't be too hard to modify the kit parts.