The bike may need to be upgraded to a more powerful motor and controller if it is used in a hilly or mountainous area.
The photos that you attached did not attach properly so I can not see them. If you could try to reattach the photos again then i would be glad to have a look and make recommendations.
If you replace the original motor with a gear motor then the top speed will be very slow. We do not carry a 24V 250W 2750 RPM motor however we do have a motor with those same specification that runs at 2650 RPM which is our item # MOT-24250X2650 sold on this page: https://electricscooterparts.com/motors24volt.html
Hi my name is naseem joseph i am from pakistan.Few days ago i buy a electric bike for my nephew age 7 years.bike have 24 volt battery 250 watt chain motor.After 15 or 20 minute running its motor get very hot and give smoke now its wiring and supply are burn.which motor or supply should i use that not get hot or burn.please guide me.