I suppose, since they're reproductions, the underframes could be steel. The rods could be faked, or even structural Doesn't the D&S have some steel reproduction coaches which look like the originals?
There's no reason to dispose of any of the current equipment, unless there's an issue with them. As someone has mentioned, most tourists wouldn't know the difference. When I rode the train last year, I mentioned to an older couple from New York that the coach were were riding in started life as a standard gauge flat car. I then realized they had no clue what I was talking about...
On the D&S, a young lady asked me why we had to stop for water so often, and as I explained it to her, I heard someone in the background tell someone I had no idea what I was talking about, that it was part of the "show" and the train didn't really run on steam anyway...
Robert