whether to add skull or not you need to understand more what that concept art is about.
I guess concpet artist wanted it to show story how from human base they did heavy unethical modifications in such extreme that only human part are bits of skin, nose and lip tissue
the way he cut mouth and replaced with mechanics suggest that whole digestive organs are gone as well as body…
Skull is there because skulls are awesome and cool ok seriously my guess is he wanted to show that even emotions and rest of humanity is just gone
btw I have to say this concept is amazing how it could talk to me, I definitely have to check out more stuff by Darren Bartley
Basically you went different way, like there is normal human inside hi-tech armor, sleeves and cloth for human tissue, proportions of massive metal parts has been shrunk down so it is on the body, not body itself.
so dont add skull, it has no meaning now, as well as add there normal mouth and lips, that are just under metal armor.
Instead of skull add there some interesting details on head shield
Its pity you didnt stay more true to concept, but dont worry about it, I also did similar mistakes for example here, where I also went for cloth instead of more mechanics my model concept