There used to be places that would put on extended shafts, even custom lengths on trolling motors. There was a place in pequot lakes, but not sure if its still there. They did rigging etc and I believe they did this.—- Quoted from CaptainMusky.
Contact a repair shop. They can most likely replace your current shaft with a longer one. I have had long shafts cut down at repair shops in the past. What model trolling motor is it. I may have an older Tour 109 with a long shaft laying around. Not sure. If these units are similar maybe we can work something out.