the retro troopers are a little bit underpriced right now so I bought two more and did this to one I already had.
Bit of a work in progress but the extra skull head from the Fist Ninja Marvel Legends fits pretty well on the neck ball joint, no alterations needed, and the helmet is about as snug as it is on the regular trooper's head
Without the web gear the body also looks considerable slimmer which I think works for a potentially fully skeletal or generally decomposed body