[paulk]

If it's just for a temporary situation, would it make sense to rig something up alongside, like a side tow, instead of at the stern? It would be simple to build a frame that the engine would attach to and then clamp the frame to the bulwark somewhere amidships. You could space out the framework and clamps to spread the loads across a long section, to avoid stress cracking. Mounting it there might help to keep the prop in the water more too.
Here's one setup: Side-Mount Outboard Engine Bracket - Small