PCI Express cores either generate an NGC or HDL source files. Despite the output file type, the options used to generate the files can be derived with only the generated files and the XCO is not necessary.
To regenerate a core it is still recommended to use the XCO; however, when the XCO is not available, then you can use the method described below.
If the core generated is an NGC, then you can run the ngc2edif command to generate an EDIF file. Then, search the EDIF for "CORE_GENERATION_INFO". This attribute will give you the options used to generate the core.
If the core generated is HDL source code, then the CORE_GENERATION_INFO attribute will be listed in the top level HDL file.