A good TIG welder could probably do it. The key will be limiting heat input so as to not weaken the surrounding aluminum
. Also not having access to the back side of the crick may cause issues with contaminating the weld. As mentioned above, you could place a sleeve over the cracked area and have that welded. The sleeve method would eliminate the contamination issue but the heat from the weld could weaken the mast. If you sleeved it and did plug
welds instead of welding the perimeter of the sleeve then you would lessen the chance of heat affecting the temper all the way around the mast but then crevice corrosion
becomes an issue. Personally I'd probably lean towards finding a replacement mast but if that's not practical then I'd sleeve it and hire a good reputable welder.