Bogies for 21mm are no problem at all,just use the MJT compensated coach bogies to whatever wheelbase you want and just use the Murphy side frames.I've not used any Murphy stuff myself as it is much too modern,but some of my GN coaches run on them as well as the nonpowered ends of various GN railcars and one end of my C Class(High Level chassis under the business end),though saying that Valencia operates purely with 6 wheelers as bogie stock was still banned in 1935.Andy