You are correct. Although Mac OS X allows you to remap the control, caps lock, option, and command keys, the fn key is not included. To do this, you'll need to use 3rd-party software such as DoubleCommand. That will let you remap the fn key to the control key.
From there, you should check out additional software to remap the control+J key etc. to emulate the left key etc. You might be able to do that in DoubleCommand as well.