And wire brushing a standard soft iron magnet with a power brush actually reduces the strength of the magnet.
Not a problem with the powder pressed rare earth magnets but you won't find them on a mower engine thats for sure.
I appreciate that rust can reduce the effecive gap and that would advance the timing by an amount so small it is impossible to measure with standard equipment and it there is enough rust it can bridge the air gap with the same effect as the legs rubbing on the magnet, which also destroys them, but iyhas to be h sort of rust you get from sea water exposure for many hours at an elevated temperature
