Speaking as a self-confessed cosmolene junkie, there are several approaches to skinning this cat.
First thing to do is wipe as much cosmo off the rifle as possible.
Disassemble it, taking the metal out of the wood.
For removing cosmo from metal, I like to use som el-cheapo carb cleaner at about $1 a can. (don't breathe the stuff. It will just melt the stuff away. When you're done spraying it down, clean as you would any other gun, and then oil the rifle as carb cleaner will get rid of all lubricants on the rifle.
MEK, mineral spirits, kerosene, diesel fuel, and gasolene will all do the same thing,as will WD-40. Use what you like.
On the wood, well, there's a bit of a dilemma.
Russians uses a shellac on their stocks that will come off if you're too agressive with the cosmo cleaning. If you're going to refinish the wood, or don't care about the shellac, then there's a number of quick fixes.
There's a degreaser called Purple Power available at hardware and uto stores that makes fairly quick work of cosmo soaked stocks.
MEK does well too.
Some use EZ-Off oven cleaner, but it can give the wood a greenish cast.
Various paint strippers like Cutz-it and others work in the same fashion.
All of these methods will remove the shellac and original wood finish too. You'll need to reoil the stock with boiled linseed oil or tung oil, or whatever you prefer.
The other school of thought is to use heat to bake the cosmo from the rifle. Some will use heat guns, or construct cosmolene melting ovens with aluminum gutter tubing and a space heater. Still others will put stocks in their ovens at low heat and let the como drip out , removing them to wipe the cosmo off at regular intervals.
In the summer, you can put the rifle out in the sun, or on the back ledge of your car's rear window and let mother nature cook the cosmo out. Some accelrate this method by putting the stock in a black plastic bag.
The final method is to "leech" the oils out of the stock by using a product from Brownell's called "whiting", or kitty litter. Basically, you fill a box with this material, insert the stock, and let it sit for a few days. The whiting or kitty litter will leech the oils out of the stock.
If the stock needs reoild, I recommend boiled linseed oil or Behr's 600 Tung Oil Finish.
Good luck.