Not sure if this will work in your situation. Back in the 1980s my dad and I bought an Imperial with an oddly shaped transom. We could not fit a motor bracket on to it, so my dad's cousin, a carpenter, cut a piece of wood (I think it was teak, but treated lumber might probably work, too) to fit the shape. We bolted the mounting bracket through the wood and transom and sealed up the edges. Ran an 8-horse Evinrude off it until I traded in the boat about 5 years ago. Worked like a charm.