First off, I'm not associated with SJG, so don't think I speak for them, but as I understand it, even an "open source", free, and freely distributed Car Wars project would violate their copyright, and they'd be forced to come after you or they'd lose some of their own rights to CW.
It's not a matter of whether or not any money changes hands, they simply haven't put CW in the public domain, so they can't allow other people to publish, distribute, etc. any CW material that isn't licensed and doesn't acknowledge SJG's ownership of their intellectual property, in this case Car Wars.
They do license game aids, etc. if you get their approval, fill out a form or two, and include the right language saying that SJG has all the copyrights to the game.
Here's their software game aid page:
http://www.sjgames.com/general/gm-aid-license.html
I expect they might do something like this for non-software projects, but it might be crossing the line on what they might want to publish themselves someday.
What says SJG?