Advance Engine Modifications
The chosen methods of metal working for a given part are usually based on economy of scale. If you are going to make a lot of a given part it is usually less expensive to have a set of dies made and cast it or forge it than it would be if you where machining it from a solid chunk of metal.
On the other hand, if you are operating in custom or small lot sizes then the CNC machines make a fully machined part not only possible but also cost effective. There is another reason to fully machine a part: if you need or want to possibly make changes to it design later on.
So what does all of this theory of metal production have to do with motorcycles? It explains why some would make billet heads and cylinders for their custom highly modified motorcycle engines.