Ok I run mags on two cars. Magnesium is porous to a degree.
I
oil my mags in two stages. The
oil penetrates and seals the magnesium.
Second my mags have tractor type valve stems, they need to be snug or they leak to.
Third mag is tricky to weld. It can be done. If the crack is visible find someone with experience to repair.
If the crack appears to possibly be a seam from the molds you may be ok with the above.
Best of luck
BTW running bias tires on mags no tubes and no air losses. 👍🏻