This archive contains a folder named Midi Manager, but I'm not sure if it's actually midi manager itself or support files...
I haven't tried it on a 68K Mac, but I mounted the cubase disk image and read the accompanying text file... it doesn't appear that there's any serial number required. I think the "protection" consists of having to boot from the original floppy (even when installed on a hard drive).
I'm definitely going to try to get this set up on *something*... it would be pretty cool to do multitrack stuff on a compact Mac.