Thanks for writing in! We have a few options for repairing this patio door handle set. If you are only looking to replace the key, you can purchase the 82-322 Key Blank and have a locksmith make it compatible with your existing key lock or you can purchase the 82-310 Key Lock which comes with a new set of keys. There is a video tutorial below with instructions on how to install the key lock.
If you would like to replace the entire set, you can use the 82-086 Legacy Keyed Handle Set. Your mortise lock appears to be the 82-265 Multi-Point Mortise Lock, which is sold separately from the set. You'd also likely need the 82-249 keeper since your existing keeper may not be compatible with our replacement mortise lock.
Take a look over each of these items and see what you think!