I really like the random drone idea that can repair your vehicle on the fly. (No pun intended) That's a much better solution then Jeff Gordon's XS racing did back in the day with their weird morphing cars.
However, rather then limiting speed, what about something a little... different.
Drones, as we know them currently, can be programmed to do... well... just about anything. And I could easily see them being made to not only repair a racing vehicle at speed, but be able to keep up with and predict all of those vehicles movements. However, having the drone just fly in at complete random seems a little silly as these tracks can be rather long and crazy and asking these things to just tail the cars around the track just doesn't seem viable to me for a very good reason. Fuel/Battery life.
As drones go right now, they need to be light. Very light. With such a low weight requirement their just isn't a lot of room for cramming battery's into them... never-mind a portable fusion laser welder... thing... yea, I just made that up... just roll with it.
This means that they would need to be kept on a charging station of some sort to have the batteries topped up at all times for when they would be called in. This also means that depending on where you were on the track, it could potentially take a rather lengthy amount of time to reach you. This could be very bad when you are in the heat of battle right on the ragged edge of keeping your machine in one piece.
So how could we solve that problem, still have dynamic repairs with drones (cause that sounds awesome), and still limit the player in how to go about repairing his vehicle while still potentially being penalized for it? Simple... Have the drones mount to the car and have the drones have limited battery life and also be destructible.
Imagine that your car just got hit by a missile and you are in a bad way and need a repair. You are down at the very limit of what your car can take, but your drone(s) are still in good condition. While trying to catch back up you launch your drone and it starts carrying out your repairs. The battery on the drone is only good enough to repair 50% of your vehicle though. This means that the drone MUST re-dock with your vehicle so that the battery's can be recharged. Depending on which vehicle you have and what upgrades you have done to your drone (thinking that in this case they can be upgraded as well) you must now either be cautions and wait for the drone to recharge before deploying and getting the other 50% repaired, not wait for the full recharge and launch it again and get done as much as it can do depending on its charge level, or do a tactical flip and launch another drone from the other side of your vehicle, and get the other 50% repaired from that drone (depending on if your vehicle is equipped with more than one drone and how they are mounted to the vehicle.
Now this is how things can get tricky. Let's say you launched your drone from one side of your vehicle as this particular vehicle is equipped with 2 drones (one per side vertically speaking) but during the repair, your vehicle flipped over. There is now no place for your drone to land when its batteries run flat and if the situation is not dealt with soon (by either flipping the car back over or launching the second drone giving the first a place to land) that drone dies and is lost for the remainder of the race. This means that upgraded mounting points to give the drones more accessible places to land would be a must at a cost of some aerodynamics or handling... or something... perhaps drone size/battery life.
You also would have to be tactical about when to launch the drones, however, as these repairs would not be instantaneous. Reason being is that the drones can be damaged and again, taken out and destroyed. Now, we can make a safe assumption that the missiles will only lock onto the cars and not the drones as they are so much smaller, but large explosions and machine gun fire... or in the strange instance... another vehicle landing on it... could destroy that drone and also take it out of the race. This means that if your car is fairly damaged, you may want to wait till you are out in clean air to deploy your drones as the other racers will see them as a prime target. Once the drones are down... you have a very limited window to make your final run and survive.
I think this could add an awesome element to the game adding that extra little thing for you to think about. You could save weight and go for no drones hoping the increase in speed and handling will be enough to put you out in front, or you could go in armed to the teeth with 8 or 10 drones on your car and have almost instantaneous repairs allowing you to be much more aggressive with your driving style at a cost of speed and handling from weight. This would make you rely on having to take everyone out of the race by force as it would be far more difficult to keep up with the other racers. Also with the potential to being able to upgrade your drones and your vehicles as an added bonus could lead to some interesting decisions being made in how to tackle the race and what your strategy is.