Well it looks like from the lens diagram that it is a YA2-3495-000 knob. It may be a different part between the older and new version of this lens. I also don't see any easily available replacement collars either unlike some other lenses. I did read somewhere that canon does not want the customer removing and re-installing the this particular collar themselves. Sounds like you will have to call canon and order directly through them.
I do not think you will find this screw. You can do one thing, search for a broken one same as yours and buy it to replace the screw. Manufacturers of these do not make and sell extra screws sadly. I do not think there is anything you can do.
