If it were my engine i'd remove the spark plug, enter the engine with an endoscope and look around the valve head for normal equal tan carbon color deposits around the perimeter of the exhaust valves. Also look for a overheated valve condition having a burnt white with possibly a slit from being torched and eroded away condition. If even tan you might have a chunk of carbon holding the valve open (fat chance!), possibly a warped valve. Check valve stem tip of having extra not wanted clearance, valve hanging open?
Investigation with a cheap ebay endoscope and laptop a quick easy and cheap DIY process. If you didn't drive the snot out of it or a lot of miles in this condition you didn't damage (torch) the valve seat possibly only requiring a light seat dressing plus replacing one bad valve. Add your (free labor) plus gaskets your up and running again and not too costly unless it's a high mileage about near death engine to begin with?
I would rather remove and replace ten pairs of 4.0 and 4.6 litre push rod engine heads than do another 5.0 litre engine AGAIN! Got suckered into replacing a pair of them on a LR4 a friend paid $750 for a LR4, a nice really clean looking low mileage LR4 with bad heads. Located two good used heads dirt cheap, new tyres, brakes including gaskets and fluids, he's into his LR4 at almost $1,800 total investment. A future wash when parting it out next expensive failure, a not if but when?
BTW, sure hope no anti freeze or burning oil were emitted out the exhaust possibly coating and destroying both those expensive Cats?.......~~=o&o>......