For the inner side - remove the rust and try to sand it as best you can. I hit it with a wire wheel to get rid of as much rust as I could.

You'll want this to be as smooth as possible. This will need to fit back into the housing and be able to freely spin with as little play as possible (this is a cheap shredder though, near enough is good enough!)

On the middle part - same deal, remove the rust (this part is less important about being perfectly smooth)

For the end, this part needs to fit into the end part of the housing.

If you hit it hard enough in Step 11, you'll notice it has mushroomed out a bit. You'll need to grind this back down so it can fit in the end housing. Keep grinding and testing until it'll fit and can spin.

I used an angle grinder to get it close then a hand file to get it closer and sandpaper to neaten it up.