The reflection / normals data is per face, so when creating a new face you need the correct data or it will appear completely matte with no reflections. There's also a chunk of per-model data that in some way links with the per-face data, so it's not just a case of copying random data from another model's faces either. Using data from random faces would also end up with a very distorted-looking car as the reflection data would be for faces that should be at very different angles and positions.
It's possible, with a ton of hex editor work, to move around the vertices of existing models to get a decent-looking result. That's quite limiting though, so only really allows for variants of existing cars.
The restrictions on models are quite tight too - maximum of 255 vertices, and one 256x224 texture for the whole model including all LODs. The palette of the texture is selected per face as it's drawn, with a limit of 16 colours per palette / face and 16 palettes overall (with three reserved for specific uses).