different advance on the ignition, I am guessing with the 28 one on the 21 bike with PGM III your ignition will be retarded.

When I put the 21 flywheel on the 28 I got advanced timing, not sure how the rest of the ignition will affect it on the 21. Maybe if you have complete 28 it will be fine?????

Personally if you have it available I would swap the flywheel and pickup coils etc from a 21 back into the 28 engine, then everything should be matched to your PGM.