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Was there any other damage when it came loose? The cam bearing could be damaged and possibly a new cam seal and cam belt required if it moved about a lot when loose. Fitting the sprocket to the cam is usually done with the intermediate cover removed so that the cam can be restrained with the flats on the back while the nut is torqued. There is a sprocket gag that can be used to get around this. If you remove the front cover you will require the gaskets.
