---Just...Cubase.
Problem solved.
Thanks to everyone who helped, especially Leosadvs, Luddite and Porter.
Simply downloading dtools75.zip from here, which creates a bootdisk with a minimal system 7.5 with no full-install required, did the thing. Cubase2.5.1r3.img expanded itself onto the desktop and inside its icon was the Cubase executable with no protection, just straight ahead into the program.
For the romantic professional musicians out there....
Enjoy Cubase on the Macintosh Classic!!! ;D ;D