I spent alot of time with Jesse Leibowitz, the former curator of Olympia. They kept as much original as possible, however you are correct that the 8" guns are fiberglass and the 5" are not hers.
They came up with a new kind of epoxy that was able to repair the waterline rust/corrosion which was/is very effective. The ship no longer sinks at high tide.
The engine rooms were restored during the 2010s, along with the epoxy hull repairs.
I haven't been connected to the museum which maintains her since 2020, and Jesse left before then.