Kinda funny that we both had Ron Howard and Yul Brynner on our cast lists in some capacity.... :lol:
Actually Ron Howard I got from your list because if we're picking from any era theres just really nobody else I can think of offhand who could capture the look and personality of Jimmy. I'd just want him a bit younger mainly because I enjoy it more when characters like Jimmy, Robin, and Billy Batson are portrayed as very early teen rather than late teens, early 20's.
As for Yul, the guy just looks like Brainiac to me. It's just very very easy for me to envison him with the electrodes on his head.