You might call around and see if the Saiga-12 is indeed verboten; it's based on an AK receiver, yes, but it's in sporting configuration from the factory and (obviously) in 12-gauge. Converting it back to look like an AK involves grinding off some rivets among other things; that's how they're legally imported into the US, as non-sporting weapons from Russia are banned from import here.
As for converting an SKS or somesuch, it'd take a ton of time and money - you'd need to find an SKS-M, and then swap out the barrel, the bolt and hog out the magwell for Saiga-12 mags at the very least. All of this would be custom-made stuff. You'd then have to fudge with the gas system, enlarge the ejection port, etc. I highly doubt it'd even be possible to make a functional repeater - you might be able to craft a single-shot out of one, but again, why bother?