I looked up a 53cc twin engine on line (hobby king) weighing 1477g = 3.25 lbs, and comparing to a G-62 with eng, muffler, Ign & mount weighs 5.125 lbs, for a 1.869lbs. addition weight. This additional 1.869 lbs would reduce th front cowl eight to 3.28 lbs, but you would have added an additional 7/8ths of a pound to the plane.
So in this case, the math shows positive and negative effects without the hours of mounting then re-weighing. Yes in Engineering we try to do most of the work on paper first to make manufacturing efficient.
For you the airplane builder and flyer, the object would be to save you time, money and auguring out the plane and finding out it didn't work. A little work on paper can save you hours of trial and error efforts.