Read the specs of both of the appliances. Usually, a manufacturer will only warrant a cooktop over a wall oven if they are from the same line. Additionally, the oven will be lower to the ground than one in a range, and the costs for doing this will be MUCH more than just buying a range. My suggestion would be to install a slide in range, paying particular attention to the needed side and back clearances for emergency landing space. Most islands, unless they are very large, are not really big enough to successfully house the cooking zone. In addition, the needed ventilation will also be MUCH more expensive.