Can they be fixed and put back up?
Yes...But it is a matter of complexity and cost.
Yes. It is a full heavy and careful project, but it’s getting done. Aluminum Overcast is in progress now. Some of these have thousands of hours now. A huge number of them never saw 250 hours before being wrecked, shot down, lost in weather, damaged so badly they were cannibalized, etc.
Nobody ever dreamed they would be flying 80 years later with several thousands of hours on the airframe. It wasn’t even a consideration as they built them. The B-29 prototype was already flying before Pearl Harbor so they knew the B-17 would only have 5 years of front line service if that.
Astonishing how they are still going. Takes a lot of effort.