Actually, having a good look at my incomplete one, you just need to put the robot head, and the arm plates and the other accessories ( gun and missles) but pretty much it's a decent Opop really, cept its got no bot head and them accessories. Ratchet is still good not too tight but not loose.
Removing the head,

gray piece 90º up and then push the head forward to slide it out

do reverse for putting it back on

here's some pics
Front - the opop names smudged off a bit

Back - no bot head

Elctronics still work - put batteries in just for demo

and as you can see the usual chrome wear

here they both are, side by side