The oil cooler isn't a nesessity. Yes it comes with one from the factory, but there's countless numbers of turbo bikes running around without them.
The oil pumps in the hayabusa are a bit on the weak side as it is, and to put a cooler as high up as the sub frame would put a lot of stress on it. If your dead set on running one, get a small 2 or 4 row unit and mount it under the rad like the stock unit.