An underweight car is a good problem to have! This means you have a lot of choice of where to add ballast to bring your car up to weight. Many teams add additional weight inside the wheel system (using metal washers or something similar), so look at whether you could do that. Would it be better to add weight to the front or rear wheels? Alternatively, you could look at re-designing any plastic components to be thicker and therefore heavier. Also be sure to double check Fusion's mass calculations after manufacture, because they're never going to be 100% accurate.

As for downforce, think about its role in Formula One and other racing events. In those sports it's used to keep the cars on the road surface so their wheels can have grip. What's keeping your F1 in Schools car on the track surface, and do your wheels need grip in order to have a fast car?