There are two issues:
1) making the rifle work with standard mags. This is actually very little work that honestly anyone should be able to do. It involves filing off a small amount of metal from the mag catch, drilling and taping a hole for a bullet guide and then screwing the bullet guide into place with some loctite. A very easy DIY job.
2) Having the gun be legal with mags installed. The long and short of it is that when you put a mag holding more than ten rounds in that saiga, the gun can not have more than 10 countable foreign made parts. A stock saiga rifle has 14 thus you need to remove (and generally replace) 4 parts. There are lots of ways to get there. A pistol grip conversion gets you there if you use a US made FCG (parts), a US made stock, and a US made pistol grip. This is another simple and very worth while DIY project.
The linked to guns are interesting in that at least the lower two would appear to rely on US made magazines for their parts count. I think that is stupid to do. They have all kinds of CAA parts on them. I am not an "ak traditionalist" by any means. However, there is a difference between making functional additions and adding tacticool parts. Personally I'm not a huge fan of the CAA parts on those guns. There are PGs I prefer, stocks I prefer, and handguards I prefer to those CAA pieces. Also I would really want to know whether the brake on the top two guns is an AK 74 style brake, which is nothing more than a lookalike muzzle weight that increases flash and blast, or a real deal 74 brake. I would not be shocked to learn it was the first. If so, why? I'd much rather have a $9 A2 bird cage than the lookalike fake 74 style brakes. And if I'm not going for a traditional look there are IMHO better muzzle devices than the 74 brake anyways.
I don't know the price of those linked to guns, nor what a base saiga is selling for so it is hard to compare the relative value of the linked guns versus a user assembled rifle that would to my mind be a superior weapon.