I do a lot of cross-platform integration. This is based on my experience. If you know you're going to cover multiple platforms from the beginning, you will do a lot of things to make that a lot easier.
Most of those would be written to target a graphics framework (like OpenGL) for the visual elements. Then, for each platform there will be a host that OS-specific things like file & network access. You can also use libraries to abstract that for you.
So the amount that needs to be rewritten depends on how similar the new platform is to the old ones. Going from PC to iOS would be about 40%. Going to Nintendo Wii, maybe 70%.
Unless the platform is particularly unusual, most of the hard problems were solved in the first release, adding additional platforms is fairly mechanical. So even if you need to replace >50% of the code, you know roughly how you're going to do it before you start.