About the only time a magnet MIGHT be useful is right after a full rebuild with a bore job or possible a hone to pick up some very small particles left after a full wash out with detergent. But that's why you have a (hopefully) 5 micron oil filter (.000197/in) to pick out all that suspended crud. If it's smaller then that, it isn't going to hurt much.

You DID wash it out with detergent, and you did clean and wire brush all of the oil passages didn't you??

As far as picking up bearing babbit - not with a magnet - tin/lead/antimony with possibly a copper backing adhering to a steel shell. If you get to the steel bearing shell - who cares, the engine is junk by that time and if at a track, oiled and debris it down .