If the new magnet and carrier is a neat fit into the old flywheel, and the attaching screw lines up nicely with the tapped hole, I think I'd much rather use that and have it rigidly and permanently attached. The only downside is the balancing job, which can be fun anyway. To find out what it takes to balance it, just weigh the old bits (magnet dust, plastic surround, and cover plate, but not the screw), then weigh the new bit (magnet and surround all in one bit, but not the screw). Subtract the weight of the old stuff from the new stuff. You will then have to make up a small mild steel plate to screw onto the opposite side of the old flywheel, in the recess there, that weighs the difference in those weights. Of course you will have to drill and tap the opposite side of the flywheel to mount that small steel plate - you can't rely on glue alone, someone could get hurt.
