I want to boost 1.2 litre petrol engine of my hatchback. What are possible ways for it except adding a supercharger?
Since the volume of your engine is fixed and your air pressure is fixed as well your only way of getting more power output is by playing with air temperature, fuel type and controller. Cold air intake would increase the mass of air that enters the engine and thus produce more pressure during combustion.
A high flow catalyst and exhaust would allow the gases to exit the engine with less resistance (also contributing to the overall power output).
Depending on your engines controller, you could reprogram it for performance though that would require a significant level of experience.
Finally burning a gas with a higher energy density would achieve an increase of power output. Burning methane for example at the same rate as petrol should theoretically give you more power as methane has 55,5Mj/Kg while petrol is at 46,4Mj/kg.