You're running into a double-edged sword in a way. You need a certain amount of speed to run cooling water through the system yet you want to cruise around with it. Fully enclosing the engine cuts it off from the air that would normally do the job for you. With going to a water-cooled system at low speeds then you'd need a vacuum-operated water pump. A water-cooled top end kit should do the job for you, a bolt-on but then consider the water pump also. You should be able to do the conversion for under $100.00 going that way.