Pencarrow
Western Thunderer
The footplates on the rebuilt Rivers were lower and wider (as I've just discovered while contemplating a 4mm version...) as well as everything else, but a U of any stripe from an N is, well, it's not straightforward.
Adam
I did start converting a Bachmann N to a U about 10yrs ago. Soon realised that there were quite a few differences. The boiler is the same though...
photos of Ns and Us in BR days in my collection which might help avoid some of the peasant traps you could be drawn in to with the preserved locos. 




































