I advise 25% of the original size, it makes for a reasonable size for a keychain. (I didn't render the pin or the symbol, but you can still print them, pin will be very little and symbol a little less but still tricky to do, consider painting the symbol instead might be better)

I advise for the body pin and shell release a 24% scale, if not, it's a really tight fit and might not work for everyone.