Regular cheapo brake fluid should do just fine.
Back in my model car building days (when I was a kid back in the late 60's, I had a paper route which provided all the cash I needed for my comic book/ Ford bubble gum/beef jerky/1:25th scale plastic car model addictions, plus the freedom to ride my bike pretty much anywhere in town) my brother and I used regular ol' Prestone DOT3 brake fluid to remove paint from the plastic car bodies when we wanted to do a repaint. Just had to make sure to wash it off well with soapy water, especially down in the door joint lines, or the new paint would peel. We never saw any crazing of the plastic. I don't see any reason it would cause a problem with a plastic airgun stock.
I'd say try it on an inconsipicuous spot first too, just to be safe.
Bryan