Hey , so once you start up the game you have to go to your profile in the main menu. From there go to the head options and you will see heads that correspond to the unit you want to play. So for example, there are heads for when you want to play an Astartes V a Necron. Not a glitch, just something that needs to be done in profile.

Realize it is a really late response, BUT, hopefully it can help other people who have a similar question!

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2838283877

Sorry to post on old thread, but this will make life easier for everyone.

Simply put the following code into the unit init:
this setFace "TIOW_MarineHead_01";

or you can use the following code for MP compatibility:
[this,"TIOW_MarineHead_01"] remoteExec ["setFace", 0, true];

Alternatively, you can achieve a random head selection by using the following code:
_face = selectRandom [
"TIOW_MarineHead_01",
"TIOW_MarineHead_02",
"TIOW_MarineHead_03",
"TIOW_MarineHead_04",
"TIOW_MarineHead_05",
"TIOW_MarineHead_06",
"TIOW_MarineHead_07",
"TIOW_MarineHead_08",
"TIOW_MarineHead_09",
"TIOW_MarineHead_10",
"TIOW_MarineHead_11",
"TIOW_MarineHead_12",
"TIOW_MarineHead_13",
"TIOW_MarineHead_14",
"TIOW_MarineHead_15",
"TIOW_MarineHead_16",
"TIOW_MarineHead_17",
"TIOW_MarineHead_18",
"TIOW_MarineHead_19",
"TIOW_MarineHead_20",
"TIOW_MarineHead_21"
];

player setFace _face;
// or For MP replacing 'unitName' with the unique variable name of the relevant unit
[unitName,_face] remoteExec ["setFace", 0, true];