MIDIGraphy is quite nice for using QuickTime Musical Instruments, also external MIDI hardware. It's 68k, not sure but it might even run on a Plus. I still use MIDIGraphy periodically. It timelessly keeps time.
http://www.versiontracker.com/dyn/moreinfo/macos/2929
http://www.versiontracker.com/dyn/moreinfo/macos/2929