Not need. You can't rewrite CPU content.
If you want to make clone, you must take new ECU hardware based on the same CPU as your.
Why you can't write the IROM, isn't it flash type memory?
BUSCON address(es)?
this what I use:
{"SYSCON", 0xFF12, 0xE604},
{"BUSCON0", 0xFF0C, 0x04AE},
{"ADDRSEL1", 0xFE18, 0x3803},
{"ADDRSEL2", 0xFE1A, 0x1007},
{"ADDRSEL3", 0xFE1C, 0x0000},
{"ADDRSEL4", 0xFE1E, 0x0000},
{"BUSCON1", 0xFF14, 0x040D},
{"BUSCON2", 0xFF16, 0x04AD},
{"BUSCON3", 0xFF18, 0x0000},
{"BUSCON4", 0xFF1A, 0x0000},