The stock is Nylon, which is one of the most solvent-resistant plastics out there. Even acetone, which eats other plastics for breakfast, has almost no effect on nylon. I wouldn't leave it immersed in the stuff, but by the time acetone evaporates it will do no harm to your stock. I use brake cleaner on plastic stocks all the time with no ill effects. Just be sure to use appropriate PPE and ventilation.

On another note, Rockite would be my recommendation for stock stiffening, unless you're planning to lay up fiberglass on your stock. Rockite is generally cheap enough to justify buying a small box even though you won't need all of it and you have a lot of epoxy laying around.