Is the mast tapering, or the same diameter all the way up? If it is not tapered, you could remove the top and shove a 24" sleeve down to where the crack is (you might have to beat on it if the fit is really tight). Then drill through the mast and sleeve and rivet them together. Riggers might have just the right type of aluminum pipe to fit your mast properly. I would still put a weld on the crack or at least use JB weld to seal it.