That thing looks like a blast! Your video says "down the coast", are you in SoCal?
You 100% want to be able to know your motor temps. If you get the motor too hot the glue holding the magnets can fail and the bearing life shortened.
I tried to find data sheets on the Heinzmann pmg 132 but was not able to find information regarding internal temp sensors or what type they were.
Depending on what Alltrax controller you are running you might be able to provide temp data directly. The SR units do not have many inputs, the XTC ones might.
External temp recording is always a plan b, but better than nothing. Require the user to pay attention and react accordingly. It's much easier if the motor temps are monitored by the controller and the system is programed to reduce power output if a threshold is reached.
Google search for "k type thermocouple". You would want one that you could mount in direct contact to the motor housing. "k type thermocouple display" will provide you a ton of products that will display the temp of the probe. Monitoring the motor case temps works but it will not tell you the whole story as it takes some time for the heat generated to be transferred to the case.
Also speaking of temps what sort of BMS / temp protection are you running?