so it seems the car is probably driven a lot which is why engine was changed, make sure your wheels are perfectly aligned, tyres are balanced and fully inflated at 30psi. These little things can be considered if you want to further improve your fuel average.
I do hope the wiring and everything else was changed along with it if it is a different type of engine, idsi engine will give you a much better average as opposed to a ivtec engine.
Do share the fuel average you get and if it improves or not