Depending on how much you have to spend on the peice of aluminum will dictate how big you wanna make it. Make the plate at least 2 to 6" s wider than the engine bracket. And at least 2 - 6" s lower than the bracket. With it being 2 - 6" s lower you wanna fold the top of the plate(at the top of the transom) at a 90. This isn't functional but I think it looks good.
Get the plate lined up in the middle and camp to the transom or have someone hold it. From the inside use a 1/2" bit and mark the plate. Remove it and drill your holes. Since you're going to remove it. I'd use some 4200 marine sealant. With a nice thick bead, I'd wrap it around the 4 bolt holes. Don't be scared to use to much. When I tighten the motor/plate to the transom I'd like to see some squeeze out the sides. Just let it dry and cut it off with a razor. Make sure to use some sort of backing brace on the inside bolts in the transom. One on the top and one on the bottom.