Toyota Repair: 1FZ-FE engine, blown head gasket, head gasket leak

Question
I have a 94 land cruiser with a distinct knock. I can drive the vehicle but I am worried that if I don't fix the problem it could lead to bigger problems. Does it sound like a Head Gasket leak? or could it be a rod connector?

Question: can I replace just one piston and rod with the engine still in the vehicle or do I have to remove it?

Answer
You are absolutely correct, it can lead to bigger problems, engine knocking noises are never good news, a blown head gasket normally does not cause this noise so I would suspect that there is a mechanical failure, possibly a connecting rod bearing, is there oil in the engine at the full level? Does the oil look dark and does it smell burned, if so then that is an indication of oil that has broken down and lost it's lubricating ability and caused bearing failure.