Its the whole gray piece and the robot head that's missing. The head can be taken off using some force.. not excessive and there's a trick on where to press so you don't snap the gray piece when trying to remove the head. You actually slide it forward while the gray piece is 90º up and it should come off easy without the need for force actually
I just tried it with my complete Opop now to see if could be done. Ill take some shots and post the pics here. You don't have to dissasemble the electronic cannon parts for this simple transplant.

Honestly though from your description of that figure , THAT is a Junker more than what my spare is. Mine just appears to be semi complete compared to that.

Erm thinking ahead though, how does $15 + postage costs sound to you?