You can make any material two sided by clicking the "2-sided" button on the material editor, it's just below the preview spheres on the right side.
Anyways i'd rather suggest to use the full engine bay and stuff from forza, so that it looks closed but without a waste of poligons(applying double sided is equal to physically doubling polygons on export).
Plus, in case you exported also interiors etc with it, but they're invisible, that's because of loading order, you got to register(add) the new textures on your rpk and then properly set the loading order of your mesh/es accordingly to the .tex file/s that come with it/them.
With so much drama in the V-S-T, it's kinda hard being Bigg B-O-double-S-ninetythree