It's a bit different, but not markedly. The main facial structure difference is the shape of the nose, and, more importantly, the fact that Mission's face is positioned higher than the default Twi'lek one. Mission's larger head wrap also makes her face, particularly her forehead, look smaller. You could conceivably tweak the shape by altering certain vertex positions.
A larger problem is that it is mapped significantly differently, beyond a case of simple arrangement, so it's not a trivial matter to convert the texture.
Another major difference is that the generic Twi'lek head model doesn' have goggle or mask hooks. Mission wouldn't be able to wear masks or goggles in-game unless you use her actual head mdl.
Plus Mission has the extra 3D modeled emblem in the middle of her head wrap.
Okay I'm still having a bit of a problem... every time I use mdlops to rename missions "p_missionbb01" texture to "p_missionh01" it strips the model of its animations... could someone tell me how to get around this, please, this is VERY frustrating.
I use a hex editor to re-name those fields in a .mdl file. Specifically I use XVI32 to edit (it's freeware), and HexCmp to compare 2 .mdl files (it's shareware with a trial time limit).
http://i463.photobucket.com/albums/qq351/redrob41/Tut-replaceMissionBB.jpg)
Once you open the .mdl file, click the Replace button (circled in red). In the dialoge box, click the Text string option, and enter the name you want to change in the top box (P_MissionBB) and the name you want to change it to in the bottom box (P_MissionLK). Then click the Replace all button. Save.
You'll then need 6 tga files for Mission: P_MissionH01 & P_MissionBB01 will be identical; P_MissionHd101, P_MissionHd201, P_MissionHd301 & P_MissionHd01 will be her dark side transitions.
AFAIK, this is the only way to make it work, since you need to keep the same number of characters when renaming. P_MissionBB & P_MissionH have 11 & 10 characters respectively, so that's why it has to be renamed to P_MissionLK (also 11).
You don't use MDLOps to change the texture. You need to hex edit the binary MDL. You need to replace it with something using the same amount of characters, so something like p_missionbb99 would be the way to go.
You can still use mdlops to do the re-naming, but you need to change P_MissionBB to P_MissionLK.
Once all that's done, you'll still need to edit the uvw map to change the position of her lekku.
I hope you can get it to work now. If you continue to have problems, PM me, and I can probably whip up a modded head model for you.