It is simple, now just upload the game to Aminet with a readme describing the licensing scheme the author has now set, and some email just so that people are able to thank him.
It is dead easy, the most difficult part has already been accomplished by you
BTW, there seems to be two different cdrom editions of this game. Both are available on Magix FTP. So, in the end, it is an easy task: just add a readme with the latest ISO, compress, and upload it to Aminet.