I've got a 55lb thrust on my boat and it's not worthy in any kind of wind, I need to upgrade.  Can I replace the motor and shaft and keep the petal, main wiring and battery connection, or do I need to replace everything?  If I can just replace the motor, how easily can this be done and what am I looking at for price?  Thanks.

you will need to replacing everything. to go higher that 55lbs you will need to go to 24v. You will also need another battery.