Since the primary function of the "doughnut" is to spread the loading around the perimeter of the hole, you can do another option: Assuming you can extract the "doughnut" without challenging the watertight integrity, use aforementioned hole saw to cut two new doughnuts, each one half the thickness of the original, or better yet, one larger than the original. Cut the doughnuts in half and thoroughly seal all surfaces with epoxy. Insert the first, wider doughnut in two halves around the throughhull base, using 3M 5200, followed by the second, smaller pair, rotated ninety degrees, also using 5200 as the bedding adhesive
. You can then screw through the upper doughnut into the lower, not penetrating the hull. The following day, when the 5200 has set up, tighten down on the throughhull nut until it is secure against the top doughnut. You can remove the screws and fill with 5200 if you wish.