Use a high powered sander to sharpen it. In our village a man comes round once a month with a megaphone attached to his car and a huge generator in the back. We bring him our tools and he sharpens them for us. Looks pretty specialist!
We had bought our sickle from an antiques market for fairly cheap - it was attached to a long pole which made it more appealing front he bramble work we have to do - but they'd painted it black with a fake silver streak as if it was sharpened. The metal was chunky so a hefty amount of sanding had to be done.
here it says to use a file, so I guess that'd be the best option by hand.
http://homeguides.sfgate.com/sharpen-sickle-82398.html
Good luck!