This story comes from the very last volume of Getter Robo Āḥ and is translated by good ol' C-K (Who translated Shin Getter Robo and UFO Robo Grendizer). He's also translating Getter Robo Āḥ, so that's great news! A million thanks to him.
Funnily enough, I got the script for this the weekend Shin Mazinger adapted this story. The design of Mazinger Z in Shin Mazinger is based off of this work, which is cool, since Ishikawa's deisgn is great.
Mazinger Z: Relic of Terror by Ken Ishikawa