If there are no other issues with the motor, I would opt to replace the head gaskets. If you can do them yourself... which is quite a bit more complicated than the standard engine as far as the timing components go. Be a good time to replace the timing tensioners and guides as well. And would be the cheapest route doing it yourself ($250 or so after new gaskets, intake gasket, head bolts, timing components etc..) If not, a used low mileage engine should run you around 1200-1500. Up to you, but some may opt for a 99+ 5.4 swap. Again, used low mileage 5.4 should be around the 1500 range. There are great write ups on this site that can walk you right through it. Either way, it is not something that will have you back on the road by tomorrow.