Fburgtx is right. With milsurps, there is no "new in box." Your best hope is "unissued" condition, which basically means it had been rearsenaled. Classic Arms still has some "excellent" ones from Samco. but the true unissued condition ones are pretty well gone (that's what I obtained, but I got mine about 2.5 years ago).
Century won't do much to a Yugo SKS- they're simply an importer on those. It isn't like the AKs, where they are assembled from parts kits. The Yugo SKS is imported whole.