Both Barbarbic! and Cepheus Atom use the same underlying rules so kit bashing them would breeze. Sure, you could do it with Cepheus Atom alone but I think Barbaric adds a few things to make it a bit more interesting. So how would I do it?
First, I’d combine the skill lists: Combat, Lore, Physical, Social, Sorcery, Super Science! (Replaces Technical), Stealth, Survival. Use the Traits from Barbaric! with a few tweaks here and there and use the character advancement rules from Barbaric!
Moks as a race would be easy. Just use the Apefolk from Barbaric! And of course, you could do a Mutant race or use with some minor modifications the existing races in Barbaric! Home brew some Super Science! vehicles and relics. That’d be pretty easy too.
I might take out the Contamination rules for player characters since that doesn’t really seem to be a thing in Thundarr but those tables would still be handy to make weird mutated monsters. Of course, strange mutagenic pools or areas might make an interesting trap or encounter area but not as a constant threat over the characters’ heads.
And speaking of mutations, the thought has crossed my mind to add yet another skill. Psionics. Once again a super easy fix. Just look at the Psionics under Beneficial Mutations in Cepheus Atom. Since it would be its own skill, I’d slightly change the mechanics for the Skill Throws; Minor: 6+, Medium: 8+, and Major: 10+.
And that’s pretty much it. The only thing left is, “We Ride!”