Can I put a different motor in it?

Your motor is your batteries. You might need new batteries. 36v deep cells require regular maintenance. You could have a bad cell and that's all it takes to keep you from the other side of that hill. Always replace all batteries at the same time even if only 1 tests bad. Batteries produce toxic fumes. Batteries can overheat and explode! Always wear protective eyewear when near a battery. Batteries need to be maintained regularly. Any signs of a battery leaking requires immediate attention by a qualified technician.