as a mechanic by trade I want to throw my two cents in.
When you say machine I would take that as planing the bottom where the gasket mates to the block. (it would be a nightmare to “machine” the entire head from scratch).
the reason most would not even attempt this is because when a cylinder head warps, it warps along the entire thickness of the head (this includes where the cam rides or even the rockers if not overhead cam) and when you only plane the lower surface then the upper is still warped. this leads to broken cam shafts, broken cam carriers, bent push rods etc.
you need to flatten (un-warp) the entire head, top and bottom. then you need to surface the lower mating surface to accept the gasket. this is done with a true flat surface (plate steel) some shims (usually a few thousands thick) and heat (oven) to flatten the head back to close to true. Then when it has cooled and you measure to satisfaction you can use a true flat surface (same plate steel) and a large run of sanding paper to manually condition the surface.
check youtube for some video references on DIY surface cylinder heads.
here are some links to get you started.