Thanks for your suggestions.

I have already begun to work on it based on a mixture of Concept 1 & 2 (Blue helmet, flat face plate with ridges). So far it's turning out better than I thought but there is still a lot of work ahead. Re-shaping the Antennas was super hard but sanding the face plate flat was relatively easy. I can predict however that adding the ridges will be a challenge. I also have to adapt the head to the ball socket of the base toy - That has proven hard but I think that I have thought of a way to do it.