I received the new Mattel Green Arrow figure today.
So the first thing I did was rip his head off!
I put his head in boiling water and carefully pulled it from the figure.
The head is attached by a neck-plug, like the original mego, but also with a post. The head appeared to be glued to the post. I don't think there was anything on the top of the post, except glue, as I don't feel anything rattling around in the head. Although it was difficult, I was able to reattach the head, so I think the post and glue and just probably to meet modern toy requirement that the head doesn't come off easy. After reattaching, it look just like new, so breaking the glue didn't seem to cause any damage or alter the appearance.
The big news is that the neck plug works great on vintage Mego bodies!
Here is the head attached to a Mego type-2 body. It is a good fit. The head can turn normally and doesn't appear to have much of a gap.
I think this was all planned. Keep the Mego fans happy by having swappable head, but glue the head on to meet safety requirements.
His outfit and head work great on a vintage Mego body:
I proceeded to remove the neck post from the new body. The head still fits fine but may feel a tiny bit looser. However, by removing the neck plug I made enough room to put a Mego head onto the Mattel body.
I put the EMCE Klingon head on the Mattel body. You can see the neck plug in the upper right corner. The hands and feet are also easy to swap out.
Here is the Klingon-Arrow all dressed up:
The new hat fits great on the old head.
Here is the new figure wearing old (repro) boots.
Here is an out outfit on a new body:
Seems to fit fine.
Here is the Green Arrow head, hands, and outfit on a Castaway body with green Castaway boots.