He realises this he is doing it because he likes them and wants to ride them, he isn't in it for profit. UK law won't be an issue with such an old bike we can prove the dates of the frame and engine and this is enough to put it back on the road, Road Tax is free and its MOT exempt, but it will be MOTed to avoid any issues if stopped. He has the correct frame engine, gearbox, part of the front forks, the rest will be manufactured or fitted with the nearest we can find matching parts. It's not being built as a show winner just an old bike to be used. Luckily many bikes of that era used the same parts suppliers so we should be able to find some bits. A newer Enfield doesn't interest him as much as he likes 20's - 30's style bikes, he would love a flat tanker 20's bike but they cost a fortune.One problem might be (don't know the UK laws) that if it is not original, put assembled with bits ancient and modern, it will not be classified as "Vintage" and will not be allowed on the road because of air pollution and/or noise. Also, making a large number of alien parts fit and work together properly can be technically very difficult and very expensive. And in the end you will have a bike that is worth nothing. Sorry to be negative, but I've done this kind of thing most of my life, starting with a 1955 AJS 500 twin, in the early '70s.
Why not buy one of the current Indian Enfields? I'm sure it would work out cheaper and you'd have something to sell on eventually.