How to replace broken chainring that is missing a tooth
Just buy a 104 BCD (Bolt Circle Diameter) chainring with the same number of teeth (probably a 42 tooth on this model).  Try searching for '104bcd 42t' on google.  

Unbolt the chainring bolts (you may need a chainring nut wrench to hold the bolts while loosening or tightening them like the Park CNW-2C) and then put the new ring on and bolt it back up.  You may need to remove the crank to get this on, but usually you don't have to worry about that for larger outer chainrings, so I think you'll be able to skip that step.