I hadn’t heard about a flathead engine being any more susceptible to sitting than an OHV engine, but I’d top off the water and make sure there’s oil on the dipstick, pull the plugs and shoot a little light oil or WD-40 in the cylinders and then as Steve said, turn it over and get things moving around and limbered up again. It may or may not turn fast enough to show a little oil pressure on the gauge as it is spinning. Once you’re sure all is well, replace the plugs and start it. Keep an eye on the oil pressure. Let us know how it goes. Good luck!
Jon