I think you have a couple options.
One is AutoRealm, i have never used it but i think it can do six flags type maps. Plus autorealm is free. Another open to consider is offering it as a job here in the map making requests section (all the way at the bottom of the forum list.) I would imagine something like what your looking for wouldn't cost too much.
Other wise, GIMP is a free program if you wanted to try the hand drawn method. I'm not sure if there is much else for programs that would work. I'm sure some one else could offer some better advice.