The internals are essentially identical.

The fuel system needs to be adapted to handle the natural gas though. Looking around, it appears that there are vehicles on the road that can switch at pretty much the push of a button.

I just found this article: http://news.ycombinator.com/item?id=4360904

It explains the process pretty well. It's really a shame it's so expensive when you really don't need that many parts or modifications.

I converted a car I found in a junkyard to work off propane when I was a student in the 90's with nothing but hand tools.