I have had very good results with rare earth magnets and high quality 2 part epoxy (Marine Tex
http://www.marinetex...epoxyputty.html or JB Weld)
But make sure you start with a very clean, no paint surface, surround fully the magnets with the epoxy, maybe even a thin layer over the top of the mag, and let the epoxy cure fully before running the car.
I have heard of some SM guys that have had issues even doing this and they built a simple 2 piece collar and clamped it on the shaft and had 2 or 4 of AiM threaded magnets attached (evenly for balance).