To do it right, the head probably should come off the car. As long as you have the OEM head bolts, and not aftermarket studs, you should be able to pull the head w/o removing the motor from the car.
A tapped-in insert may work well, but IIRC there's not much material in the head where the plugs go, so I don't know how well an insert would work. You might be best off just getting your hands on a used passenger head for the WRX motor. They shouldn't be all that expensive, since everyone wants the STi ones.
The good news is that it doesn't look like anything fell into the motor. The bad news is that it looks like something hit the spark plug!
I hope it wasn't pushed out of the head by the piston somehow!