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...