About the only solvent I would rec is ordorless mineral spirits. For this to work, you would need to immerse the stock in a tray and let is soak. That will get some of it out. Unfortunately, we are entering the time of year that makes it difficult to use the sun for this. A very good way to remove cosmo from wood is to "bake" the stock. I have used a cheap metal cabinet with a heat source such as a trouble light. Put the stock on its nose end against the inside of the cabinet. With a 60w trouble light on and the cobinet closed, it will warm up quite nicely. Cosmo will ooze out. Bake and wipe...repeat until you stock getting cosmo.
At this point, begin to rub in BLO. Once an hour for a day, once a day for a week, once a week for a month, once a month for a year.
The solvents in the BLO will remove more cosmo. You will be left with a pretty clean stock. Afterwards, you may wish to coat it with a reddish or clear varnish (stock) or lacquer. Stay away from poly urethane. Tung oil is nice, if you get pure TO and not TO mixed with PU.
Millions of SKSs were made in a dozen countries. They continue to be used all over the world by local militia, tribemen, rebels, and home guard units. Many were used in the Balkans in the 90s.
Have fun.
Mahatma