Jeff has a perfect response. Have to stay on top of the bearings, which can be done by habit of spinning each by hand before or after mowing.
I'll add that only mowing when it's completely dry and frequent enough to not bog the motors down. At least adjust the deck height and mow twice if you have to... These motors are no longer being made, so only so much stock left.
There are two options for re-adhering magnets. A special motor magnet epoxy is available and very expensive. This is what most motor shops will use that are willing to take the risk of the work. Key items are cleaning the shell up leaving no rust behind and then baking them while the epoxy sets to keep moisture out. The second option is plain old JB Weld. I know of a few that have good luck with it, but I think the cleanliness of the shell is key.
I have been keeping a stock of shells until it was worth it for me to purchase the magnet epoxy and do a whole bunch at once, cleaning each with a blaster and applying epoxy in batches.
I also keep a small stock of armatures ready to go, but I'm no longer mowing with an ET. Time is critical with a 2 year old boy to chase around.
Good luck,
Chad