Short version: Saigas are imported in "sporter" configuration and are different enough from AKMs or AK-74s to make them importable without conforming to the UCC 18, 925(d)3 which defines what can and can't be imported as "sporting."
UCC 18, 922(r) says that you can't build a rifle out of foreign made parts if you couldn't have imported it under 925(d)3.
So, the trick is to beat the definitions of a "foreign made" rifle by having no more than 10 foreign made parts. Replacing those parts of the Saiga that make it "sporting" with "non-sporting" type parts violates 922(r) UNLESS you replace enough of them with U.S. made parts to make it a "US made" rifle. Then it can be as "non-sporting" as you want.
So, if you want your Saiga to use high-cap mags, (along with adding a bullet guide) you'll have to replace some other stuff to get below the "parts count." Stocks, gas piston, muzzle device, fire-control parts, etc. all count. Ironically, the easiest way to make a "sporter" Saiga 922(r) compliant is to do a partial "conversion" to the AKM pattern using US made parts.
Regular AKMs are generally imported as "parts kits." Foreign made machine guns that have their receivers (and now barrels) cut into unusable bits, so they can be imported as simple replacement parts. Then they are rebuilt on U.S. made receivers by one of our AK building companies and the usual "922(r) compliance" parts are installed. (There are some other options for this...but that's generally the way.)
-Sam