Well, other than adding a pump and possibly resealing the tank it could actually be used now. Only problem is that the tank is made from brass or bronze and the plumbing is nickel and copper all of which are toxic to fish, although I'm not sure about nickel.
Don't clean the piping. The oxides have sealed the metal.
If you do want it shiny, have a musical instrument (horn) company do the polishing and seal with a synthetic varnish.
All the faucets in your home are made out of brass. Many of us have lead soldered copper water pipes. The water coming into my home has more copper in it than I'd get from the piping. A lot of older homes in cities have lead lines from the street to the meter. Even that didn't cause a lot of "old time" fish breeders from having issues, decades ago. Metal oxides aren't just rust.
It's also possible to have the inside of the pipes plated, but that would cost a bunch.