The CIA had good success with putting Hellfire missiles on Predators. A Hellfire is an anti-armor weapon, so it might not be up to the job of hitting a fighter. It'd sure do a number on a fighter, though.

If a Hellfire isn't up to the job, it shouldn't be all that difficult to put a couple Stingers under the wings. It'd take a bit of work to know the Predator is under attack, swing around to get the Stinger IR seeker on the target, and fire the missile, but in some ways, that isn't as difficult as shining a laser designator on a target to hit it with a Hellfire. Hmmm, I wonder how hard it'd be to mount the Stinger to fire backwards...