AirVenture must have 500 engines on display... real ones, mockups, and a few which are pure fantasy. Most are the same old thing in a different color. However, here is one you do NOT see every day, a Gnome Monosoupape 100 rotary.
That's a rotary, not a radial; The crankshaft is fixed to the airframe, while the crankcase and cylinders whirl around with the propeller. Not cheap, but perfect for your trusty Sopwith, or with an Oberursel data plate, an early Fokker. They're available brand new from Classic Aero Machining Services of New Zealand. For more information, visit www.kipaero.com