It depends how much trouble you want to take. It's best to start with a complete rear from a 917K (eg Fly or NSR) which takes care of the chassis tubes, exhaust, gearbox, lights. The tubes need to be rearranged a bit. The two boxes are simple rectangular shapes but they have metal banding around them. I built these from scratch thus:Having had a good look at the rear piece, Daniel, it could be easily made from scrap plastic. Wouldn't take long, either, as it's not complicated.
Then there's the oil radiator attached to the tube holding the rear lights!