If you've installed ISE, the source code for many cores, including FFT, is in directory vhdl\src\XilinxCoreLib (in version 8.2i).
If you don't yet have ISE, the free WebPACK probably includes source code too, but I'm not 100% sure of that.
What type of LCD?
If you need to store data, and RAM is available, why not use it?