I asked the same question a while ago. I believe the differences are:

1. Carbs + TPS
2. Flywheel so the timing is different. Don't use the HRC card with the MC21 flywheel - too much advance.
3. Depending on the year, the oil pump might be different. The newer MC21 and MC28 have an extra oil feed.
4. The gear position sensor is different if I remember correctly.