The Kimberwick can be described as a short shanked leverage bit so a short shanked Western should work the same way. An Argentine bit such as these: Discount Horse Supplies - Horse.com with the mouthpiece of your choice should work.
They are not only short shanked but also, having a high purchase to go with the low shank, give a low ratio as a Kimberwick would.
The bits I linked to can be used two ways. The first way is at the end of the shanks and the second is close to the mouth piece. When it's close to the mouth piece it has very little, if any, leverage and can use a direct rein.
You need to give more information/background when you post questions. Both I and Iride put a lot of time and effort into our posts to you, and it was all wasted. If we would have known that you were asking because you were showing in snaffle bit classes and she was already trained to a western curb, then our answers would have been much more simple and quick. When you don't give all of the information needed, most people will just make assumptions and go from there. I don't like to play 20 questions on a message board.
Really though, if she's that young, you shouldn't have her in a Kimberwicke. You should go back and train her properly to a regular snaffle. A kimberwicke is a more "advanced" and harsh bit, not something I would ever ride a young horse in. I only use my kimberwicke on horses who need retraining to go back in to a regular snaffle, or on my Anglo Arabian who goes better in it on trails (he uses an egg butt snaffle for all arena work, home and away).
If she doesn't like single jointed bits, try a french link or bean/oval mouth. A full cheek with a copper roller in the middle is a nice bit too.