Parts is pretty much parts, the lightweight engines have to be smaller somewhere...
Good example is my new G62...By making my mount thinner, leaving the ball bearing mechanical advance of and using syncro spark on the ignition, and making a lighter hub, the weight is down to 4 lbs...I guess if I could drill out the crank and cut off most of the cylinder fins the engine would lose a few more ounces, but why ? Smaller fins would bring cooling issues, a lighter crank would bend easier or possibly break..A thinner mount would not be as resistant to bending..