The R-GT regulations state that minimum weight of the car is its power in Hp multiplied by 3,4. If this Abarth has 300 Hp it can go up to 1020 kg. I don't believe it's achievable (just like cca 1330 kg for Porsche 911) but for sure it allows to make the car as light as possible without any additional balance.

By the way I wonder how FIA determins the actual power of the car...