I've had a front magtek straightened recently, it was only a very minor dip in the rim... but I will check both my magteks for cracks before repainting them... Its quite easy to do yourself using a dye penetration kit (cleaner, dye and developer in 3 aerosols available from any engineering suppliers or through the internet). You need to take care to clean the wheels properly both before and after using the dye to get good results, but its definitely not beyond the diy mechanic. I've seen kits available from about £15...