I wound up using McDonald's toys based on the movie Cars instead of those Marx ones. You could just slap some plasticard onto them if you really wanted that particular look. But aye, there's plenty of die casts out there that would do too, and being wrecks its not as if people will be looking too closely anyhow.
I do have some Marx cars of my own however. I haven't touched them with any paint as I thought that it could be possible to make a mold of each of them and fill them with clay to make copies. Its not something I've had the time to follow up on, but if I ever bother with that Hirst Arts Molds stuff again its a possible.