Can I install Spaces and Exposé on Lion?
This is impossible because Mission Control / Spaces / Exposé are actually controlled by the Dock. And you'll certainly have no end of trouble if you try to transplant the Dock between OS versions.
You can't, it's Mission Control all the way. I'm actually quite happy with it, and made a pretty seamless transition. What features are you missing? You can do everything with Mission Control that you could do with Spaces and Exposé, with the sole exception of Spaces in rows. You can even assign applications to individual spaces; that option is in the app's dock icon now.