You'll need Notepad++ to edit the huge .bsp files in text form. For e1l1.bsp in Breakthrough's Pak1.pk3, for example, search for "human/" and you should find the first human model on line 192,067.
From there you can change the model .tik to your desired one, but you'll have to compare origin coordinates in-game to change the correct AI actor. Example is below:
Code:
{
"classname" "ai_german_afrikacorps_soldier_headwrap"
"scale" "1.0"
"model" "human/german_afrika_private_headwrap.tik"
"testanim" "idle"
"angle" "180"
"$targetname" "farleftspawn1"
"origin" "-1264.00 4912.00 256.00"
"target" "t222719200"
"type_idle" "idle"
"type_attack" "turret"
"type_disguise" "salute"
"type_grenade" "grenade"
"gun" "Mauser KAR 98K"
"sound_awareness" "100"
"noticescale" "100"
"fixedleash" "0"
"enemysharerange" "0"
"waittrigger" "0"
"balconyheight" "128"
"accuracy" "45"
"ammo_grenade" "2"
"disguise_range" "256"
"disguise_period" "15"
"disguise_level" "1"
"gren_awareness" "10"
"#playerseek" "0"
}