[mjjust]

I can think of a few issues you would need to consider.
1. Weight rating - I have no idea what your Opel weighs, but I suspect it is quite a bit more than a cart and it may exceed the safe limits of the wheels.
2. Most carts (in the US anyways) use a 4 inch x 4 inch bolt pattern. I think a 4x100mm may be off by 2-3mm, but it may not make a difference.
3. Speed - Carts are designed to operate at 15-20mph. Wheels designed for carts may not be designed for the speeds seen in an automotive application, and may not be safe.
4. Width - I don't think I am too crazy about narrowing wheels. I've seen some disastrous results when doing that to motorcycle wheels. I don't think I can trust them.
5. Cost - If 13 inch wheels are available, why go to a 12"? By the time you buy the 12"and have them modified for your application, you have probably spent more than buying a set of 13" wheels, tires, and probably lunch. I would think that it you would have more choices of tires in a 13 inch as well.

I like out of the box ideas, but some times they are more hassle than they are worth. Cool little car, looks like an early Nova.