Hi wee_man
Good question, pretty much any high performance bike head is made using high pressure die casting. This forces out more of the air bubbles that can plague an ally casting.
And I imagine it would be more than a two piece mould. Heads are a complex shape, even before machining, and the mould will probably have a lot of movements in it.
The head will have some initial machining, then the valve seats will be pressed in, the cam ladder or camcaps will be bolted on and the camshaft bore will be drilled plus any final machining.
cheers,
Kurtis.