That engine will be tricky. I would suggest making a new prop driver that is more solid without the deep groove in it, so you have a place for the magnet. Another method would be to make a two piece slip ring that fits into the groove of the prop driver so you have a place for the magnet, plus you can use set screws to lock the slip ring in place when you have it set. The slip ring would be made in two pieces so you can put it on and into the groove of the prop driver. You would have two screws holding it together.

Now then you could simply build up a dam or barrier and glue the magnet into place using J&B weld epoxy. You need to make a equivalent glob opposite the magnet to help keep it balance of course. One could also simply fill in the groove with epoxy too.