Acetone works on celluloid.
You won't damage the scales if you keep the cloth moving (i.e. never let it rest in one place). The trick is to make a tight ball so that what contacts the scales is mostly the cloth and not liquid acetone. Use fast light strokes. If the end finish is not as glossy as you'd like, you can finish with a polishing compound or simply a clean rag and some elbow grease.

Admittedly this technique is not for beginners. If you have some old scales to practice on that might be a good idea.