I remember more than a decade ago that someone uploaded a DL audio link of the Super Robot Spirits concert of which one of them was the Ballad of MazinKaiser. The live version however featured a dialogue by Rika Matsumoto and Ichiro Mizuki. That dialogue featured a story of MazinKaiser as told by Akira Kabuto, the descendant of Koji. I used to have that audio file which was sadly lost. And then I found this:
Brief info and story:
I have reason to believe the said dialogue may have been taken from the radio drama version of MazinKaiser. If so, then this may be it.