Where do you prefer it to be placed on the 13 inch - just replacing the keyboard and touchpad?
Probably doesn’t make sense space wise and as @Name2 pointed out it might be better to just buy a controller. For the 13 inch you also have to redesign the complete metal input cover plate for any change. So input modularity is not really a viable thing there. But when someone brings out a Trackpoint Keyboard, you could maybe abuse that as an joystick.
This here is just a fun way to use the Input Modules on the 16 inch.