If it were me, I would dry scrub with a brush first, then if you want to clean it further add water. Those are brittle, not sure what they are made of, but they corrode very easily. I have the same one somewhere around here... Nice find!
You've been given good advice to use dry brush then use soap and water. From experience, if you want to loosen those inner workings use PB Blaster. Will loosen it up, just give it time.
It looks like you have a nice, intact cap gun there. You might be able to get it working. Good find!