As noted they have two barrel profiles and only one will accept standard AK-74 compatible barrel components. The heavy profile barrel is fairly uncommon and seems to have been mostly on earlier models that came in without mag well dimples. To find out you need to take a caliper to the barrel and check the dimensions.
Mic the journals for these barrel components to verify an AK-74 OD spec barrel:
Front sight block: .570
Gas block: .590
The AK-74 spec barrel will also accept a standard handguard retainer although you'll have to make the crescent cut into the barrel for the locking cam. This can be done carefully with a round faced jeweler's file once you mark the location for the HG retainer.
The cheapest mags that will work right now are the AC Unity 5.56 mags sold by RTG Parts but like HWI mentioned you will need to install a bullet guide and modify the mags to fit as the Saiga .223 has an oddball magwell/trunnion space that is actually the same dimensions as a 7.62x39 AK. Look at the Beryl mags side by side, on the right you will see where the front of the mag forward of the feed lips has been flat filed down to clear the locking lugs on the front trunnion. The AC Unity mags are dimensionally identical to Beryl mags here so the same step is required. The only problem here is that all of the AK-74 spec mags are narrower than the mag well and trunnion so most will have a bit of wobble even when fitted to the rifle, they will start to wobble over long term extended use. In my experience this never lead to malfunctions with Beryl mags but it was annoying enough to me that I switched to using Weiger mags, which once fitted to the rifle are the correct width for the mag well, so they will never wobble or wear in.
Back in the day I also used Tapco Galil mags which work unmodified with nothing but a bullet guide. Unfortunately they are discontinued and limited stock is available. They also have no steel reinforcements anywhere. My experience with surplus Galil mags is they were never reliable in the Saiga, I used both Izzy and SA Galil mags, same problems with both feeding too low and bolt would often ride over. Even modded them by bending the rear tab to get the seating height higher. I even swapped out the spring and follower to NIB Magpul spring and no tilt followers (yes they fit). Still no go. But the Tapcos worked 100% every time.