There are several ways to make something out of metal. The part can be stamped, cast, forged or machined. In fact, most metal things are a combination of more than one of these operations.
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 were 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 machines 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 its design later on. So what does all of this theory of metal production have to do with motorcycles? It explains why custom racing companies would make billet heads and cylinders for special engine projects. Over the years custom engine companies have learned that if you built high-performance engines for all kinds of racing, you need to understand that no one engine can fill the needs of every rider.
By using very advanced multi-axis CNC machines, custom engine are able to tailor the engine's individual parts to the performance needs of a client.
Just having a few CNC machines does not guarantee a great engine, though. The key ingredient is the years of engine building experience and testing facilities. Looking at some operations you can be amazed at the attention to details in each engine's assembly process, but also the way in which high performance engine are build in Thailand with parts custom made to design. Equally impressive is the cleanliness and organization of the engine assembly area itself. It's like a NASA lab.