Not really hard, the sprocket on the engine is held on with 2 bolts, the one on the axle should have bend tab washer's ( that's what I call them) against the bolt head. Order new ones as well, once you benmd something once, its weak and not worth taking a chance on.
You'll have to remove the rear wheel, and hub to get to the sprocket hub. Not a big deal at all if you have some tools and are confident you can get the job done.