Posting this because it looks like nobody has answered your question yet. I hope it helps.
The faction leader's skin is defined by variant meshes with both the frontend 3d model and initial faction leader being defined by att_frontend_faction_leader_rome_west.variantmeshdefinition in this case.
Each consecutive leader is defined by their att_general_(and so on).variantmeshdefinition There should be 17? of these you need to update.
Therefore to make a change you will have to change the variant mesh on all of these variantmeshdefinitons
The thing you were refering to i think is the olive wreath so the line you would be interested in from the roman emperors variant mesh is this
incase you don't know where to put it, here is the frontend variant mesh definition done for you
Just doing this wont update the porthole pictures with your changes, but if you have porthole's set to be 3d not 2d in your graphical settings this wont be as noticable. good luck!