Sorry, but short of replacement your only practical alternative is to wrap the beam with a cover layer of wood. Building a box around the beam won't help it's structural integrity, but appearance certainly will improve. This kind of twisting is relatively common and usually is not dangerous. If there is a question about the holding power of the beam, request an inspection by a structural engineer. For future reference you might ask for timber graded "free of heart center" for exposed beam applications. Lumber cut away from the center of a tree (free of heart center) has less tendency to twist after installation.