You can probably take both a Japanese and English rom and create an ips with the differences.
I fail to understand how that helps. Could you clarify?
To port a patch from a version of the ROM to another one, you need to:
- identify the list of changes from the patch. The patch file itself should give you that.
- identify which structures in the old ROM are modified by the patch
- look for the same logical structures in the new ROM (which can range from simple, stupid search to complex header identification and offsets calculations)
- hope that you can apply the same change, and if it doesn't work then try to understand the original logic and re-apply the logic (not the patch) to create a new change.
The diff between the E and J roms is only a partial view of all this.