I've run a bike rigged temporarily as a single speed before. Simply putting the chain on one of the middle sprockets gave a good gear ratio and an acceptable chain line. If this works out for you, then there's no compelling reason to replace the freewheel or even remove the extra sprockets. However, to get the best aesthetics and a marginally stronger wheel, you should replace the freewheel with a BMX one, adjust the axle spacers to centralise the hub and redish the wheel.