It's already flashed into the ROM; at present, I don't see a need to change it. If you really want to do that, you would first have to debug it, next to modify it, finally to flash it back to the ROM, but the flashing method is hardware-dependent. Copying will not work.
However, I do not understand the way you want to proceed. Like other PP-based players, the (fixed) bootloader does the necessary steps to initialise the device (e.g. CPU, ATA-interface and a bit more), and then reads its main firmware from the encrypted mi4-file. This mi4 file is what you have to build completely new, based partly on the available Rockbox sources, and partly with new modules you have to write on your own, dependent on the YH-920 hardware. In order to understand how the hardware works, it will indeed be helpful to debug the bootloader.