Porting Delphi code should be quite painless. The Delphi mode of the compiler tries to mimic Delphi as closely as possible. This mode can be enabled using the -Mdelphi command line switch, or by inserting the following code in the sources before the unit or program clause:
This ensures that the code will still compile with both Delphi and FPC.
Nevertheless, there are some things that will not work. Delphi compatibility is relatively complete up to Delphi 7. New constructs in higher versions of Delphi (notably, the versions that work with .NET) are not supported.