I recently saw an article about Honda (being the world's largest engine OEM) working on an aero engine.
They did some research and the optimum layout was a flat 4 pushrod design! but with modern fuel injection technology, lightweight components FADEC etc. The basic layout it seems is still valid, but technology in general has moved on and should be encompassed.
Can't remember any figures so will have a root around.
http://www.eaa.org/communications/ea...tcm_honda.html
http://world.honda.com/news/2003/c030304.html