(this answer only works for Linux users, sorry)
There are two issues here.
If you have a problem with the previewer user interface provided by Okular, I think there is not much you can do. You would probably need to fork Kile entirely to change that.
If you have a problem with the PDF backend (poppler) you can compile a different PDF backend for Okular, that in turn will be used by the previewer inside Kile. As far as I know there are two alternative PDF backends for Okular:
1) muPDF backend: which will completely replace poppler and kind of works (I use it on a regular basis)
2) Cairo backend which will only replace the poppler subpixel font rendering with the one provided by Cairo
Your distribution may already provide packages for the alternative backends (for example, Arch does).
Since you've not stated what your specific problem is, I can't really tell if changing the rendering engine will solve your problem.