This subject comes up quite a bit on train buff forums, I'd bet a google search would come up with several good ones that explain it in detail.
The GG1s had an insane amount of miles on them before they got retired, they did indeed have frame issues throughout their lives, and you'd have to pretty much replace everything but the carbody to run one today. By the time you were done, the locomotive would be suffering a huge case of Theseus's paradox (think of the 'George Washington's axe' argument).
It's a shame as several GG1s were saved and a few are in 'rotting away' condition today...
