As the saiga is an import you need to make sure you comply with the Federal 922(r) US parts compliance law.
I don't think you are even ok with modified mags and definitely as soon as you add that stock you are committing a felony unless you replace the correct amount of US made parts at the same time.
ATF has decided that the ability of a semi-automatic rifle to take a "large capacity military magazine" (LCMM) is an unsporting feature, so possession of a modified magazine could be a problem.
The way it works (if I recall correctly) is that they are imported in a sporting configuration (including the 10 rd mag, no muzzle attachment, and no pistol grip), as soon as you replace the mag with a 30-rounder it is considered "similar" to a weapon banned from import and it becomes subject to 922r compliance (meaning no more than 10 "imported parts").
Edit to add: this is important, I think that if you have a unmodified saiga and a 30 rounder modified to fit the saiga you are breaking Federal law. Give the modified mags to a friend temporarily and add enough US parts to make the saiga legal ASAP.
Most people seem to get around that (from a quick read at
http://forum.saiga-12.com ) by using a us made 30 rounder, and us made disconnector or gas piston.
That won't work for you however unless you can get hold of a (non-existent?) pre-MA-ban US made 30 rounder (and dispose of all non US made ones).
Here are the 14 imported parts on a saiga, you need to replace 4 of them to use a non-US 30 rounder, a buttstock with pistol grip, or a muzzle attachment:
Receiver
Barrel
Trunion
Bolt
Bolt Carrier
Gas Piston
Trigger
Hammer
Disconnector
Butt Stock
Forearm
Magazine Body
Magazine Follower
Magazine Floor Plate