If the void in the underside of the valve will move over the hole which it looks like it may do then you could get steam/air escaping down the exhaust.
Can't see the exact shape of the hole but if it is just half an egg shape then it is going to get smaller as you machine the port face so may clear the valve.
I would do as Stew says and machine up a thin piece of CI about 1mm thick, cut the ports in that and retain with a couple of CSK screws that fall under the steam chest edges, smear of JBWeld or Araldite to seal. Steam chest studs will hold it firmly in place on assembly