The best solution is to change the radiator but if there are budgetary constraints, then upper tank can be replaced. It has to be done very carefully and there will never be peace of mind because it might leak again any time.
The radiator guys will charge you half the price of a new radiator for just the upper tank. In my eyes, it is a bad deal.
Replace the radiator and enjoy peaceful motoring. Take Xulfiqar's advice.