Providing the mast is water tight? Or the reinforcement "jacket"?
I have no personal experience welding aluminum ... if those cracks can be cleaned and filled with weldment, absolutely. I imagine then it would make sense to really clean up those cracks. Grind a nice, clean channel in their place with a smooth radius at the termination.
It sure looks like that step insert though is a cast piece of carbon steel. That bothers me. I'd much prefer a billet of stainless milled to shape.
1981 C&C 32