There's a shareware sequencer called MIDIgraphy which should run on at least the Mystic, IIRC. If you hunt around ebay and the download sites you might come up with an old version of Logic or Cubase or another sequencer (Dr T? Mastertracks?) that will run on the SE. Try the website Shareware Music Machine at hitsquad.com

A serial port MIDI adapter for Mac should be around $10 on evilbay