These kind of fires that start during renovation of old buildings are often caused by lead burning (welding) where the sheet cannot be beaten or welted or the heat from halogen lighting near tinder dry timber. Led lamps are more common these days though. A stray cigarette end or the sparks from cutting/grinding could do the same job
We nearly lost one of the Queen's pads not so long ago when hot works on the roof ignited the timber. It's easily done.
Apparently, the structure of the building is ok so I'd imagine it will be restored to its former glory.
I hope the Vatican will use some of its vast wealth to fund it rather than relying on hand outs from the general public.

Rust never sleeps !