Rust air ducts..

Hello everyone.
Recently I discovered that my rovers air ducts from the heater box are riddled with rust and scale. It extends from the windshield air vents to the passages underneath the plastic dash under the speedometer. All the way into the box section unbove the foot well/ pedals. The sills seem to be in decent order with little patching needed. Lots of surface rust behind the rear seats on the bulk head. Here is some photos. I'd like to try an get at it an replace the metal. Does anyone have advice on tackling this situation? I'd like to try an save my p6. As she drives so well and has a lot of life left.