Welcome to the Forum
There are quite few people out there each with their own take on model engines. Some make the most exquisite replicas, almost jewellery, some experiment with different designs of their own, some love castings, others prefer barstock, there's a space for everyone.
I would suggest a barstock engine from plans as a first build. When things go wrong, as they almost certainly will, you lose some time and a cheap piece of metal rather than a more costly casting.
I have not moved into CNC yet but I'm sure others will be able to help.