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.
Be mindful of raising the temperature too high when using power sharpening tools. I think for this tool it isn't as critical as a knife edge, but raising temp and cooling down improperly can be a challenge for steel edge tools. When using a power tool to sharpen, one could change the properties of the steel which can alter the hardness or brittleness of the steel. I prefer hand files for cutting tools such as spades, shovels, hoes, etc. I would also use a hand file on a hand sickle to create a primary edge and a stone to create the cutting edge. Just some additional info for your consideration.