GG-1 frame is a little odd, the nose rounds down at both ends. The only way you MIGHT be able to get away with this is to take a scale MTH PS-2 GG-1. In which case you would already have a scale GG-1 with PS-2 in it. If perchance you found someone with a trashed shell for an MTH PS-2 GG-1 loco you could TRY to figure out a way to attach your shell to it. It is extremely unlikely that the screw holes/bosses will match.
Good Luck!!!!!!!!