The only "official" procedure would be to take the camera apart and replace both sides of the broken part with an unbroken example of the same part.

If the camera were a little bit newer and worth a bit more I would contact a Canon Service Center for an estimate before trying to do anything myself. But another used 450D can probably be had for not much more than the cost of shipping the 450D both to and from the Service Center.

Unless you are experienced and very skilled at repairing compact electronics, though, taking the camera apart and reassembling it has a very, very low percentage chance of success. Even if you are a master at repairing things such as smartphones and other very compact electronic devices, without a service manual that lays out the proper procedures in the correct sequences, it can only take one tiny mistake to irreparably damage something like a powerboard or a cable connector that could render your camera useless.

If it were me I would try to do this in stages without taking anything else apart.

Use some sort of hard setting plastic to build up the center a bit (being very careful not to glue the center nub that needs to rotate to the surrounding parts that don't move). I'd try something like Permatex "form-a-gasket" from an auto parts store. Use the kind that hardens, not the kind that remains flexible after it has cured.

You only need to build up a millimeter or so. Before it cures place the dial on top of it (with the slots on the underside of the dial lined up properly with the tabs on the switch) and press it on just enough to form the shape of the nub in the center of the dial onto the top of the putty.

Remove the dial and let the putty cure for a day or two. Use a very fine small file, perhaps a fingernail emery board, to insure the material you have added doesn't prevent the center shaft from rotating freely. Then use another drop or two, applied with a toothpick to the nub in the center of the dial, to glue the dial back onto the shaft you have extended. Let it cure for another day or so before testing it to be sure the dial will rotate properly.

Another option, if everything else stays seated in the current condition, is to use the camera as it now is. You'll have to be extremely careful about keeping moisture, rain, and sand out of the camera. But the Rebels aren't that weather resistant anyway, so that won't be a lot different than having the mode dial in place. After turning the dial you can always confirm what mode you are in by using the "Q" button to bring up the Quick Control option on the camera's rear LCD screen.

Good luck.