There is quite a selection of 24v trolling motors.
e.g. An 80lb thrust model like this would do well. http://www.westmarine.com/buy/produc...lb-24v-62.html
If the race organizers didn't spec out the top trolling motor specs, considering their built in waterproof design, this is truly a bummer. They're more than up to the task for propelling this canoe.
If you must mount your existing motor I would seek out a suitable drive shaft, flex coupling and hull fitting. This will be tough to track down as most boats today use trolling motors. They are ideally suited. I prefer Minn Kotas as the motors are powerful and most efficient. Their propellars also perform well and deflect weeds & debris.