any machine shop can cut a groove in the head for an o-ring.

Motoman is not quite correct, the o-ring groove needs to be cut a certain way along with the sealing surface as the o-ring does not actually do the sealing so much as providing a barrier.  Take it to someone who know's what their doing.

Send it to Carl (Ice racer) or somebody else who has a good rep.

wouldn't they need to allow for gasket too, squish? What do they do, haunch out the dome? IDK.

You may need to get a thicker base gasket but a good builder will be able to tell you exactly what you'll need to do to fix all of it correctly before you even send it to them....Call Carl.