I'm going to say incorrect PAL... What board was used for the conversion and was the PAL already a G PAL or was a G PAL replacement installed? Or did you pull up the pin on an F PAL and run it to the J9 pin on the multi?
Also did you make sure to jumper the memory clear pins next to the battery and completely clear out any remaining encrypted code?
Jumpers are potentially the problem but usually that causes no boot - look up Mitsurugi-W's install guide on YouTube and ask the person having the problem to confirm everything in that video