Do you know what the processor is? The binary is not much use without some idea of what it runs on. Maybe you can find out the locations of key data that might be tweaked and not worry about the executable code. Can you just write new data? Is there protection against making unauthorised changes? Are things are checksummed? Encoded in some way? etc, etc. Can you re-load the original binary if you make an error?
You might just turn an expensive ECU into a paperweight.