Well, I don't know about maps, but AFAIK, with images, making something too similar to something copyrighted (and if there is no license information, one should assume it is "all rights reserved") is still a "derivative work", and needs authorization.
http://commons.wikimedia.org/wiki/Commons:Image_casebookhttp://commons.wikimedia.org/wiki/Commons:Derivative_workshttp://commons.wikimedia.org/wiki/Commons:Fan_artI still hope that making something that resembles an existing map, but has got some differences, and is done from scratch, may be ok, but maybe the difference level may be an important factor.
Anyway, if there is a way to contact the original author, one may try to ask his permission to make a "remake" of it, or even to get himself re-publish the .map under the gplv2 license. If he answers "no", I suppose it is better to focus on other maps...