I think you have it.
The Pegasus had four ports like the cylinder in question (OK 3 visible) and an R-1820 should have the valves in line with bore, not offset. The cylinder in question has almost 1/2 the head missing but the two remaining valves/springs look closer to what is left of a 4 valve head.
An R-1820 should have a 9mm larger bore. Granted there is a lot of corrosion but if it lost 4.5mm per side there would be a lot more stuff (fins) missing.