Because the bus weighs so much more than the mobility scooter, the engine wouldn’t be able to move the wheels. The bus would just stay still. But not sure whether you could power a small car. If it was light, you might be able to use the engine
If you had the right gearbox, you could move a bus, but it would be very very slow! (gears can amplify the force of a motor, but you would need to turn a very big gear with a very small gear on the motor, so the big gear would rotate very slowly).