My only advice is to find one that the magnet is not pressed in.
I used to run one of those in our old YFZ race bike and one time when I was changing the oil I discoverd that the magnet was missing. I spent a week pulling everthing I could off the motor, trying to find it. And ultimatly I was able to pull it out with another magnet on the end of a stick, thru the drain bolt hole. But I was on the edge of pulling the motor and splitting the cases. Our bolt was aluminum and of course the magnet part is steel so it was just pressed into the end of the bolt.