Plasma cutters don't normally need many CFM of compressed air. A very quick Bing suggested 4 or 5 psi is typical at about 70psi. That's really cheap electric compressor territory. I'd imagine any petrol powered compressor will provide at least that.
It was just a quick "google", don't know how accurate it is. Somewhere in the loft are some old scouting books inherited from my father and I remember seeing semaphore as one of the skills taught to scouts, I wonder what todays youth would make of it?