The thumbhole stock on my Saiga is an IZHMASH factory part. The old stock is removed which is easy with the removal of three screws, two on the top, one on the bottom. The old stock is wiggled out or tapped lightly to get it going. The new stock slips right in. I tapped it a few times on a carpeted floor to seat it all the way. The two screws on the top are replaced. It does require you to drill two started holes on the new stock, which is incredibly easy. The whole installation took me 15 minutes, and I was taking my time. I bought my stock here:
http://www.gilbertsguns.com/Stocks/Saiga/Russian+Saiga+Skeletonized+Stock They were good folks to deal with.
Since these guns are coming into the US now with these stocks, and since you are swapping a foreign part for a foreign part there are no 922r concerns.
I have found that the "pistol grip" portion is wide from front to back, but in practice not nearly as wide as it looks. I find it to be a comfortable reach. I have larger ahnds but I don't see this as an issue except possibly with small hands. The cheek piece rotates out of the way and locks in place in two positions. The padding and material is quite nice and appears as if it will be very durable. The cheek piece sets my head to high for the stock sights, but for optics the height is nice. But then again, it can be rotated out of use The POSP mount is excellent and can be acquired numerous places including eBay. This is the one marked "Belarus".
Just one more thing about the Saiga. I say this owning a .223 and a 20 gauge, and in 30 minutes I'm leaving to go pick up a .308 if it is still there. IMO, these guns are possibly the best value in the gun world today. They are built rock solid and represent the best built Russian "AK" in existence. For guys who want they can modify them out the whazoo, which looks like a blast, or for a guy like me they can be modified with ease without conversion for shooting. Or, as in the case of my 20 gauge, they can be left stock as a sporter, an option that to me has merit, especially for a shotgun. One way or the other the prices will rise on these guns drastically, and if the antis have their way they will be banned. Lets face it, if it comes down to it and any gun bill gets legs, the gun industry and lobbyists will compromise, and it will most likely take the same form it did in the past with imports taking the big hit. I see the future of the Saiga very doubtful and predict that if one of the gun bills gets legs the antis will use the Saiga as their poster boy the same way that they used the Streetsweeper and USAS-12. An evil Russian wolf in sheeps clothing, and, a magazine fed shotgun with ten round mags available. They won't be able to resist. My thought is to get them while you can. They are already starting to get scarce, from what I hear.
On the other hand, if someone like Fred Thompson gets into the White House (sorry for the plug, but he is 100% pro-gun) the Saiga will continue to be available and at worst you'll have one heck of a rifle for next to nothing.