Many maps here are CC licensed. If that is a license and usage you can use then can you find someones map in the finished section that is a CC city map suitable in layout and style and then the problem would only be to add the meteor strike to it. I assume that nobody has built a house in the strike zone. So you can just put some scorched earth into that and add the walls. If the meteor struck after the city was made then the layout around the strike zone would not have changed even if its not sensible to a city any more. Adding a meteor crater to a city map is probably a lot easier than starting one from scratch - cities being such a lot of work to do 'n all.