You can cut out templates, join them together, and make the rest of the surfaces with putty like these modelers did.
http://www5a.biglobe.ne.jp/%7Eam-kis...age-body1.html
http://www.h6.dion.ne.jp/%7Efzero/F2000-BODY.htm
Or, you can make a solid master part out of balsa wood or putty for vacuum forming.
http://www.technologystudent.com/equip1/vacform1.htm
You can also make a basic shape with polyester putty, and carve it, like you would do to make an ice statue.
If you can obtain cad data, you can go fully digital. Although it will cost a lot, you can contact one of those rapid prototyping service, and have them make a master for you. They'll do something similar to this.
http://www5a.biglobe.ne.jp/%7Eam-kish/mp4-20/page1.html
http://www5a.biglobe.ne.jp/%7Eam-kish/mp4-20/page2.html
There are so many ways to build a body from scratch, and I think it's just a matter of what kind of material you like to use, and the type of method you feel comfortable with.