You can go for Sports At which is the high end top variant of Hyunda i20 and it powered by the 1.4L, Gamma petrol engine with four speed automatic transmission gearbox that this variant offers are Dual Front Airbags, Passenger Airbag On/Off Switch, Front Fog Lamps, ABS with EBD, Electrically Adjustable ORVMs, Rear Washer and Wiper, Heaight Adjustable Driver Seat, etc. The fully featured this variant of the car has good resale value and deliver good power and performance.
For further details about the car please visit the given link as above.
Initially, Hyundai Elite i20 Magna comes with a petrol and diesel engine but a CNG kit can be fitted with the one which has petrol engine. You will have to sacrifice with power to get more fuel economy and if you convert a petrol car into a CNG car, you may loose the warranty on the car and resale value of CNG cars is lesser than petrol cars. It is always recommended to get the CNG kit fitted from the authorized service center so that you can avail the warranty services as well. Moreover, the performance will depend on how the car is driven and maintained.
Click here to find the nearest Hyundai service centre by selecting your city in the drop down list.
How much fuel goes into a car varies on a lot of factors. Now you haven't mentioned how many km did you drive post the low fuel warning light as that could also determine how much fuel goes into the car. Even though the manual says that the tank capacity is 45 litres, the amount can vary by a small margin, give or take 2 or 3 litres. Also filling fuel in the hot day or in the night also makes a difference considering fuel expands during the hot day and contracts when the temperature is cooler. All said and done, make sure you fuel up within 3 to 4 km after the low fuel indicator light pops up.