It's not worth your money my friend.
Putting a better (I read stronger) spring will surely break your gun quickly.
Crosman airsoft guns are very low quality (cheap plastic) so the body (or mechbox or whatever) wouldn't resist the pressure of a stronger spring for long.
Don't invest money in that thing. Instead, put money aside and buy a real airsoft gun when you will be 18 (if you are not 18 already).