Hi and Welcome to YT. Have you checked out the national organization
www.ytca.org, you can go to the health information section; although I don't think they post % rates per various types of conditions.
Now rescue dogs have the b enefit of being fully vetted prior to placement, and any health conditions are usually known to the legitimate rescue, and fixed if it is a condition which can be fixed.
Some common conditions Yorkies are prone to are'; 1) Luxating Patella of various degrees of severity 2) Collapsed Trachea 3) Liver Shunt. Now of these three with an older Yorkie rescue, all these conditions would likely be known if present.
Good luck with your search, and it is always advisable to plan financially for your Yorkie through either pet insurance or a pet savings account of some nature.