Hi Harry,
how bad is it? Is the head properly warped or just a bit irregular?

Turn the head upside down on your best dining room table, get down on your knees and slowly drag the edge of a steel rule across it, front to back, side to side. You'll get a pretty good idea of what you're up against and see light where the low spots are. A light dusting of talc can help here.

If there's a few thou gap then you could get it to a useable level with DIY flat grinding (I use emery paper taped to a toughened glass sheet from an old oven) but any more than that and it will need machining, as others have said.

Cheers!
Rudi