In 28mm one guy in a uniform with a gun looks like any other barring some specific details. You could swap the guns out and use them as all sorts of things, even civilians or militias depending on how you paint the clothes.
I'd suggest maybe looking at Empress Miniature's head range and using those to modernize them a little (they're in their Moderns -Misc section I believe). Then there's the Project Z range by Warlord Games like you macgarns 16s and AK-74s (though unless the base model's guns have a pistol grip, they wouldn't look like any military variant of either of those guns. Again, that may not be a massive issue).
Though why that kit in particular? I really dislike Warlord's plastics for their ape like physique with nigh on Games Workshop proportions (they're that poor that I was using Warlord Games arms on Wargames Factory bodies to represent guys wearing heavy winter coats). Rather buy either one of the Wargames Factory WWII or modern kits - fix the poses a bit (mostly just cutting a "V" shape out of the groin to bring the legs closer) and they work for most forces - sculpting on extra bits as you like.
But yeah, the US uniforms and the weaponry available in that particular kit were surplussed out to loads of nations Post-War. So its not like you couldn't use them to represent some National force decades on from WWII. Or mix in some modern gear, random camofladge patterns and bits of civilian clothing, then possibly Empress' African heads, and then you have some generic MENA militia force. They'd probably work well with The Assault Group's lines.