Thank you very much! That list has been really useful, I had tried hex workshop and madedit, but I didn't know about transelhexation, and that's exactly what I was looking for.
As for the game: I have figured out lots of things like finding the text and decoding and encoding the sound,
The problem I'm having right now is about rebuilding the ISO: If I modify the files directly inside the ISO and rebuild them with CD-DVD Rom Generator and Apache3 it works fine. But modifying a 4GB file is not really easy. So I decompressed the CVM files inside the ISO (MagicISO and Apache3 worked fine, since these files are just ISOs with different extension) and modified one of the files inside (I just changed some characters, I kept it the same lenght). I opened the corresponding CVM file with Apache3 and tried to use the "Replace selected file" option to replace the file I modified. Although I made sure that they had the exact same size, Apache3 says that they are not. If I check "Ignore file size differences" and click Replace File, it just crashes. I tried doing the same with MagicISO, but, obviously, it doesn't work because MagicISO doesn't have the replace option, you have to erase the original and insert the modified file, if I try to rebuild the ISO with that file the game doesn't start (I'm using Open Ps2 Loader via SMB so I don't have to burn the game every time I try or run it on an emulator).
Do you know any application similar to Apache3 (which webpage is now a porn website --U) which I can use to replace the files inside the CVMs with the ones I have modified? The only version of Apache that I have found is a beta (it's hard to find an application that has the same name that another famous one) Do you know where to find a previous (stable) version?
Again, thanks for the help.