The only legally compliant way to obtain these files is to dump them directly from an original physical Xbox console that you personally own. This requires a softmodded or hardmodded Xbox and a homebrew tool like EvolutionX or ConfigMagic to extract the system files to a USB drive or via FTP. Step-by-Step: How to Configure the BIOS in Xemu
The MCPX (Media Communications Processor) ROM is the first piece of code executed when the original Xbox is powered on. It is a tiny, 512-byte blob stored within the MCPX chip (the Southbridge manufactured by Nvidia). xbox bios files xemu