Since you're using PRE-RE (which the projects are not mantained anymore), so need to choose the files depends on your need.
most probably files in lua files, datainfo, and others. To knows which files needed, need to check on how to add custom item for that.
Example headgears = accessoryid, accname, etc.
Garments = spriterobeid, etc.
If you just considered about items, the skill infos might in Renewal too since its not changed to pre-re.