Well depending on how much LCL you've used, all you should need at a bare minimum is:
Project.lpi (xml setting for your project)
Project.lpr (main body code)
Units.pas (extra units)
Stuff you may have added due to making an application instead of just a program are the Unit.lfm files which are the Lazarus form data.
The rest of the compiler generated files such as .o, .ppu and .compiled are just for FPC to do it's magic for the linker.
Here is a Windows batch file that I use called Clean.bat: