how to remove screws from inspection plate that are rusted solid?

hi I am trying to remove the plate in front of the gas burner on my carver water heater but the left hand screw is rusted solid and I do not want to break it off.
Can I have any helpful suggestions as to how I achieve the removal of it and will it really matter if I break its head off as it is only a short screw and I can get the right one off.