get the "NISPACK" work and unpack the START.DAT bothe english and japanese... now sort the files by filetype... now you will see a bunch of .dat files again (both US and JPN release), grab the .dat files on US release and replace it over to the japanese one. Repack the Japanese START.DAT.
I think there are 15 files missing on US START.DAT that's why if you swap them with Japanese base game, it will just stuck with the loading loop.
Model: CECHC04 PAL/EUR 60GB SKU Console
Firmware: REBUG 3.55 CFW [DEBUG MODE, DEBUG XMB, MENU OPTION 2]
BM: Iris Manager v1.2 (Hermes Syscall8)