you get also that stability issue, by removing the old powerplant, you need to add more weight far down in the ship.
But if the engine is in better shape than we think... then a gas solution is doable, but don't know on that scale, The Norwegian Oslo class used Gas to power it's turbines...
Sixty year old turbines? Won't those have to be opened up and bladé inspected? I'm asking because I'm not sure if there is any wear issue or metal fatigue involved that should be of concern.
As for diesels, the old 1920s turbo electrics the Americans used were very heavy. Their Diesels were not as light as the German Diesels (or as good) either. I agree with you that ballast would be a strong issue of worry as well as the work spaces.