The AWB does not forbid the Saiga rifles from accepting >10 round magazines. That law is a result of the 1989 Assault Weapons Import Ban, which states that a rifle needs a certain number of US parts (5, 7, I forget) in order to use "high capacity" magazines.
So even if they made a high capacity magazine for a Saiga, it'd be illegal to use unless the rifle had US parts (less parts if the magazine itself is US made). For the price of getting those US parts, you may as well have a feed ramp installed and convert it to use regular AK mags.
I'd like to see a high capacity converted Saiga that retained the original sporter stock. It'd be...different...