You might not need the plug... Mine shoots light loads fine on the stock plug...
A plug being a solution for an under gassed gun is largely myth. If you look at how the gas system works you'll understand why,
The man who designed the original plug, gunfixer, had the following to say concerning after market plugs and under gassed guns:
This is nothing new, as no plug will make up for an undergassed gun.
http://forum.saiga-12.com/index.php?showtopic=58283&st=60 Post number 71.
I would not buy the stock you have linked to. Those tapco folders are not very good stocks and tend not to hold up even on a rifle. A 12 gauge will be even harder on them. If you want a folder there are much better, albeit more expensive, options. If you want a folder the easiest option for a stock that wont be a piece of junk, or (IMHO) look ridiculous, is to buy an internal block (available at CSS), cut the tang, and then attach one of the various folder mechanisms to the block and then attach the stock of your choice to that.
Do your self a favor and get a better stock even if it is just the standard tapco fixed stock.
The other thing that kit does not have that I prefer is a retaining plate. You can hold the FCG in with the wire pictured, or with e clips, but I prefer to use a retaining plate. it is cheap and it is easier.