One of the enthusiastic restaurant drivers once asked an engineer at Preston Workshop "Why can't we build a brand new W class?" The answer he got was "We've lost the plan."
I suspect there are more than enough drawings in the archives to coble something togther if there was the will.
However, I suspect if you built a new 'replica car', you would have to apply for a DDA exeception for it. One action group filing an objection would sink the program. They would be required to build a fully accessible tourist car.
By 'rebuilding' an existing car, it's simply a safety upgrade of an existing car. Even if not much is left of the orginal by the time they are finished.
I have seen a modern Flexity derivative styled to look some what like a 'heritage car'. While it's shape and colour what some one remiscent of a 50s bogie car, it was 100% low floor. Bombarlier probably charged a pretty hefty price for the modification to the platform to get the 'heritage' look.
The East Germans became experts at these sorts of 'rebuilds'. Under offical Eastern Block rules, the Germans had to abandon their tram rollingstock industry and buy trams from the Czech's. They would take cars into the workshops and they would come out completely 'rebuilt'. As a result of needing to keep this pretense that they were just overhauling cars, they kept turimng out quite primative single truck vehicals quite late. (It would have been a bit hard to explain away a short single truck motor car coming out as a long bogie car!)
The Sydney Tramway Museum's Berlin TZ69 cars are an example of these - built in 1969 to an old design but holding the serial number of a much older car. Offically it was a rebuild of the old car, not a new build.