Any AK-47 mag will fit, but they won't feed unless you install a bullet guide. I believe promag makes some specifically for Saigas, but as usual, they get horrible reviews. SMG, formerly Surefire, makes Saiga specific mags (no BG needed) and they function great. They come with a 5 year warranty, but they are pricey.
I wouldn't rule out a conversion. It vastly improves the rifle. If you do keep it sporterized, I would recomend getting a Tapco trigger for it as the stock trigger leaves ALOT to be desired. There is a Tapco trigger specifically for people who don't want to convert.
Actually, a standard AK mag will only fit into the older Saigas without modification. After the first few years they started making the mag catch a little bigger so that regular AK mags wouldn't lock in place any more. I have one of the older one where the mag locks in place but it even with these the mags will not feed because of the bullet guide issue.
There are a couple of Saiga Specific aftermarket mags that will fit a stock Saiga but reliability may vary. I also have heard varied things about the Pro-Mag mags.
There is one AK mag that I discovered which will feed into a Saiga and that's the MasterMolder Thermold mag. I found one of the first ones that came out at a gun show and noticed that it was designed without he little dip in the front that most AK mags have so I tried it on my older Saiga and it fit and functioned. I posted this info over at the Saiga boards and many people have been able to replicate my findings. Of course if you ahve one of the newer rifles, you'd still have to either file the mag catch or the mag latch on the mag itself to get it to lock in place.
I managed to make a US parts conversion without doing the pistol grip conversion. I had a custom buttstock made with a more pronounced grip area but that worked the same as the original ones. You just have to cut the actual trigger portion off the trigger and continue to use the stock trigger (that's one less U.S. part unfortunately). Custom stocks are a bit pricey and the guy that made them isn't in the business anymore though. You CAN get a Saiga specific forearm that preserves the original lines of the rifle.
It's very easy to install a bullet guide.
True, but once you install the bullet guide you've then modified it to feed from military style AK mags and you'd really need to do a U.S. parts conversion to CYA.