If your template supports it, you should have numeric class ids for each menu item. If you have that, or if you can add that to your template/menu, it's probably safer to just use CSS and style those menu items that way.
So if your menu item is say class 155 it would likely appear in the li line as:
<li class="active item155">
So then, again, following your structure, you'd do something like this in your CSS:
li.item155 a i:before {font-family:FontAwesome;content: "\f0ce";}
Where that \f0ce is the font awesome code for that icon you want.
If you don't have those menu id's you could fudge it with li:nth-child(x) where x is the menu item in order from first to last. So to style menu item number 3, you do this:
li:nth-child(3) a i:before {font-family:FontAwesome;content: "\f0ce";}
If you use that, I'm not sure how far back it works with browsers, but it means you don't have to override a thing.