Normally I have very little to do with modelling, but I somehow stumbled upon this...
This is actually a very simple situation. I'm assuming that the Weaponless Undead Arthas model is just Undead Arthas/Death Knight with his weapon removed (that is, no changes to the face or anything). Therefore the default Death Knight portrait would be sufficient.
So to do this, all you need is the Death Knight portrait, and to import it correctly. All you need to do is import the portrait with the exact same path as the model, and the name should be
identical, but then with "_portrait" (without quotes) on the end, before the extension.
To save you the trouble of exporting the model yourself I've done it and attached it for you, and I've even renamed it to match the model. So if your model is, for instance, "war3mapImported\UndeadArthasWeaponness.mdx" (which would be the default), then you should import my attached model as "war3mapImported\UndeadArthasWeaponness_portrait.mdx".
Hope it works!