I'm sure you cant transplant the English PSP script into the Jap 3DS game. Everything would have to be typed in manually and maybe other modifications need to be done to get the English script to fit where the Japanese script was.
Yup, the 3DS and PSP versions use completely different data types. The text box also has much harsher text limitations than the English PSP version, meaning either the game would have to be completely rewritten for things to fit, or there'd have to be in-depth hacking to either change how the font works, or add more dialogue boxes.
And that's not even taking into consideration some of the custom additions made to the English PSP version, like subtitles for sound-only dialogue.
Then there is the Japanese image files will either have to be remade into English.
That'd be the easy part of translating it, honestly. It wouldn't require any additional hacking just to translate images.