HI
I fixed magtek rims on my MC18 sp.
My expirience in magteks fixing is with a problem like you in the photo on my front wheel and in Greece a rims expert fixed it perfect but i had only to paint the rim because in the fixed area he warm it to do it safer with no cracks.
My other rim the rear had some cracks on the same area and i went it for fixing to the same person and told me that he can not weld magnesium and the only thing is a special glue for magnesium which i dont know what it is.Finally he fixed it but in the first potholes the fixed area cracked and a large piece removed.
I will never use magteks again in the roads in Greece who have many potholes just only to say i have magteks,it is silly.Only for track use