Answers:
Disassemble the plane and load it onto a cargo ship. The ship will sink lower into the water with the plane on board than it was before. The change in freeboard will translate into a tonnage loaded.
Compact the plane into as tight a cube as possible. The volume of the cube multiplied by the density of the main component of the plane (e.g. aluminum) will provide a good estimate of the weight.
Tow the plane down a long runway at increasing speed. At the wind speed that the plane lifts off the back calculation of the engineering for the wings will tell you the "lift" provided. This will equal the weight