A more brutal but simpler upgrade would be to remove the old open axel bogies and rivit and simply plug in a set of Mk1 bogies as fitted to the more recent short clerestories, Railroad Stanniers and other coaches.
This would however totally devalue the coach as a collectors item!
On the subject of a replica, I wonder how much more it would cost to reproduce the above assuming the tooling still exists. Also I seem to recall all those turned brass table lamps with pink lampshades which were different to the later triang hornby (now railroad) Pullmans.
An alternative could be to release one representing the parlour car Mary on the KWVR