1991 Ford F-150 motor knocks

With the engine knocking, sounds liike it could be a rod or valve. The easiest way to do it is pull the engine out of the vehicle and rebuilt it. With 200000 miles on it you might want to look into finding a used engine. If the engine ran fine befroe the knocking then you have the two options rebuilding your present engine or getting another one. They have crate engines that you could buy but that runs kind of expensive. The price on rebuilding your engine can vary depending if there is any other mechanical problems in the engine.