A bit of imbalance can be worked out by playing around with spring and damping rates. Obviously keeping the weight balance near where it was will be the best of solutions.
Concerning Ronnie's retirement / stoppage in Cholistan, it was due to the fuel pump not being as low in the tank as was needed. There was a subsequent miscalculation of fuel consumption which resulted in that unlucky incident.
I am personally of the opinion that one should take an older vehicle and really dive into it to turn it into a rally car. All I see is external and bolt-on internal cages in newer vehicles. Roll cages are supposed to be welded to the chassis and the body. They will only then provide the required strength.
Then of course there is the question of reinforcing your frame, drivetrain mounting points and suspension pick-up points, but that is a discussion for another day.
As for cooling issues, it's simple really, if you're upping power, you'll have to improve cooling. You can't just close your eyes and hope for the best. Often times, as @akifjanjua noted, the radiator isn't at fault, it's just the airflow into and out of the radiator which is insufficient. Go to town with the angle grinder, cut away the inner arches, put exit vents behind the front wheels, put reversed-cowled hoods like you see on over-engined muscle cars.
For most engines, the rule of thirds applies very well. Fuel energy is converted in equal parts to motive power, exhaust heat and coolant heat. That is to say, your 220 kW engine will generate 220kW of heat to be dissipated by the radiator and 220kW to be dissipated by the exhaust system. As long as you keep that in mind, you're generally safe.
