So I bought A full kit with a controller, throttle, motor, etc... And I do not know If can exchange for a new motor because of 24 Volt 350 Watt Electric Bicycle Gear Motor (MOT-24350G) ($89.95 value!) looks too small to move a 300lbs go kart? Any solutions?
If the gear ratio is for a top speed of 12 mph or less then the 350 Watt gear motor will have enough power for a 300 pound go kart as long as it does not need to go up any hills. If you want the go kart to go faster than 12 mph or go 12 mph up hills then a more powerful motor is recommended.
I notice that the controller in your kit is rated for 500 Watts so any motor up to 500 Watts could be used in place of the 350 watt motor without any modifications being made to the kit. As long as the kit's 350 Watt motor has not been installed or used then we can exchange it for its value on any other motor that you would like.