I am not a fan of welding on aluminum castings such as those found on a Hobie. There is just too high a probability that it will damage the casting in some way. This is mostly due to the heat applied during the welding process.

My advice on how to fix a worn mast base and mast step is to just buy new ones. You will get components that are set up for the proper mast rotation and will not have to worry about any damage that might have accured to the castings.

I am not saying that they are impossible to weld, but that you really need to know what you are doing when welding aluminum.