If it were me I would cut the shaft and sand it clean. and coat it with flux. Then fabricate a collar/sleeve from a length of 3/8 copper tubing, slip it overboth pieces of shaft and adjust the shaft to proper length. If the fit is too loose cut a small piece from collar/sleeve length wise (ie from end to end. and using vice grips squeese it tightly, making certain that both pieces are straight and inline and solder it, as you would a copper pipe in plumbing. I’ve done this sort of thing often. How much too short is it? I’m assuming the shaft is steel, not aluminium. Stand chassis on end making the shaft parallel to the work bench. If your soldering iron or soldering gun is not hot enough to do the job, use a propane torch on low fire. Don’t burn the house down!