while this isnt necessarily a modification, Alot of people have been looking for these maps to use in Total Conversions and there is almost no documentation on how to convert these maps from cmp/mmp format to a 3ds max readable format..
but thanks to an old GTA 2 map editor that was tucked away in the corners of google. I was able to extract them into the .cae format...
GTA 1 Maps - Liberty City (NYC), Vice City (Miami), San Andreas (San Francisco), Manchester (GTA 1961), London (GTA 1969)... (17.8MB Zipped & 394MB Unzipped)
GTA 2 Maps - Downtown, Residential, Industrial... (15.4MB Zipped & 305MB Unzipped) (OBJ Version)
As an added bonus... Here are the GTA1 and GTA London radio stations, they are converted from their original wav format to mp3
GTA London 1961/1969