You cast the Mona Lisa in gold and attach it to a stick to turn it into a sickle! It's really big and bulky, but when you swing it fast enough your new sickle cuts lots of wheat! Just don't tell anyone what you did with the famous Mona Lisa painting and you should be able to get away with this!
Now that you've turned the Mona Lisa into a sickle, you have to change the little description under its name in the Louvre. You draw a giant comma (かま) next to "Mona Lisa" and write "THE SICKLE." Then you shove it back on the wall as best you can.