When building a flathead you need to remember compression is not as important as flow! when you port a flathead just clean up the ports, no need to make them bigger. If you go Way back in the flathead section you WILL learn a lot about the flathead. Just doing a good valve job and cleaning up the ports will make a big difference on how well it will run. B&S goes by cubic inches not horse power. The 28ci goes by 12, 12.5, and 13 horsepower. The 11hp or 25ci head will raise compression but folks will debate on whether or not it helps. Some depends on what else you do, cam, carb, ports, flywheel and so on. Yes the lighter aluminum flywheel will add performance! GOOD LUCK with your build!