You could start over with a brand new engine and make it in the shape of a GG-1 and that would be much better then restoring any old GG-1. The expense and then the material needed would not be cost effective in any way. The frames on most original GG-1's are beyond repair.
The main thing about the GG-1 is that they used 25 hertz(or cycles) compared to today's 60 hertz(or cycles) electricity and they had different voltage as well. So the motors would have to be changed out along with the transformers inside, making it too costly to attempt.
Lee Fritz