I always found it far easier to just start from scratch when reproducing small parts that were originally handmade. Most of the sights, sling swivels and such were turned out by children in sweat shops back then. Just the way is was. No two are quite the same.
The sights available from NECG only bear a passing resemblance to antique sights, and are made quite differently. You'll spend more time trying to modify something that still won't look right when you're done, than just making all of it correctly in the first place.