A rather quick and dirty tool is to pick some photo's from the clay model, from the right angle and perspective, drop the visibility in Photoshop to 10% and sketch over it with a black pencil. I printed quite a few shots, and sketched during our holiday in the Spanish sun!
Basically I am looking for the right lines of the lower part of the tank-seat. This will be a fairly straight line, giving this ride its forward leaning character and the feel of a true café-racer!