Does the bolt that clamps on to the shaft have any clearance left or is it tight against each other. if you are tight you can cut a little off each side to gain some tightness.  heat the thing red hot and clamp it down,should bend to fit. Worst comes to worst you can weld it on. Not out any more than before. can also drill through the clamp and bolt it together middle and top. I would try the heat and tighten first beat on it while your tightening it. Should form around the smaller shaft. Can drill through the middle if you have to.

 As others have said a good machine shop can do wonders with it.  but if you need to be going now you can do it.

       good luck