If you are really going to do such a thing (and I shudder, being of the opinion that even the sorriest wreck should be restored to 'new'), you should know that there is a very good chance that you don't have to settle for a single-shot.
In this part of the world, old Lee-Enfields still are around in numbers, especially if you know a shop-owner who TAKES them in trade and then buries them in the back room because he makes more on a $1200 rifle than he does on a $75 rifle. Besides, he knows that he can sell them to me and I wouldn't buy a $1200 even if I could afford it: too much Scots in the ancestry! And people generally can't tell one magazine from another. The last Number 4 I bought had a fully-functional SMLE magazine in it. They are that close, really. If a fellow were doing the conversion you contemplate (and I shudder once again!) I would think that any mods in order to use 'the other magazine' would be very minor. Get it in and it will feed just fine.
Have fun!
Now, where did I put that SR-D jug? Grumble, grumble... Forty-five-seventy, of all things! Oh well, James Paris Lee did it, so I guess it's okay. Forty-three Spanish would be even better....
Hmmmmmm.....