It may be her food. When we first rescued my Jean Claude( who was found by the roadside as well, by the way), I fed him canned food, and his poop was like nothing I've ever smelled! I switched to a natural dry food and things got better. This probably sounds silly, but when I can, I stand by with my scoop and flush the poop down as soon as he goes. There's also a great product called FON (feline odor neutralizer) that most vets sell or you can purchase online that is a fantastic odor killer. You can spray a little over the litter and it will kill the smell. It's a bit pricey, but works on cat box smells and lasts a long time.
NOW- having said that, I will tell you that it may just be that she's one of those cats who has stinky poop! My Rhett has this problem and I've just had to learn to deal with is as best as I'm able. Good luck and congrats on your new kitty!