The Royal Enfield Big End floating bush is soft "White Metal" as used on normal car big end shells. Personally I'd contact Russell Motors or AMC classics and buy a proper new big end assembly.
The original crankpins were made of two pieces, pressed together. There were later made pins made from one piece that had an inherent problem, which has been addressed to my knowledge.....But worth asking and or getting advice about.
Big ends are very expensive but then I understand that's the way to go.
A friend suggested that he would use an aftermarket heavy duty roller bearing and ensure he would fit this to proper tolerances. I am however a little apprehensive.
Has anyone tried this sort of a modification?