If the fit is still somewhat snug, you can get everything very clean and dry, apply a drop of super glue to the bottom of the shaft and using a hammer and some sort of punch (the socket of a mini screwdriver with interchangable tips works well) lightly tap the shaft in until the top of the shaft is about flush with the top of the impeller well. If it is too loose for this to work, a replacement motor is the only repair option, part 6055.100.