Reverse engineer the ROM ?
You initial approach is good, but maybe go a bit further, before you claim "
debug menu already found".
Maybe have a look at all graphics and textures 1st, to maybe reveal unused ones.
Explore all the sound files and maybe reveal unused ones.
But I bet you arent the 1st one looking at this ROM with a hex editor. ;-)
Here, have some fancy text, regarding debug menus, inside of various versions of "Ghouls'n Ghosts")
tcrf.net
tcrf.net
Good luck and feel free to share (and change your topic title maybe)