Lots of stuff out there with the label of " Cosmolene" .
I never worked out if it is different stuff , different age , different conditions or whatever . I do know that what works to remove some stuff doesn't begin to touch some of the other stuff.
Mineral spirts works well in many cases , brake cleaner works ok when soft but fails badly on some of the harder stuff.
I bought a cheepy steam cleaner that worked wonders on a old Enfield.
Try a bunch of different stuff is the best advise I can give you.
You spoke of a "new" stock. Don't toss the old one , history and all that stuff. Any original wood no matter how beat up is what the historical value guys are interested in. You might be surprised
how old wood cleans up , try to avoid sandpaper . If you decide to go with the new wood at the very least toss the old stock set in the back of a closet or gun safe for the future.
Also , we here really get off on before /after photo sets . Seeing the gun all cleaned up is cool but take the time to take a photo series with all its crusty nasty before looks