The only reason to move the engine backward is to leave room for a much larger pulley on the mower drive shaft, without it hitting the small pulley on the engine crankshaft. By the time you put the small pulley directly on the crankshaft instead of it being moved 3 or 4" backward by the distance between the gears in the reduction unit, you've effectively moved the engine pulley forward 3 or 4". It looks to me as if it would hit the bigger front (mower drive shaft) pulley unless you moved the engine backward that 3 or 4" plus another couple of inches because of the larger front pulley.
Yes it does change the center of gravity, but that is undesirable. That is why you have to carry out Step 1, to ensure the mower won't want to tip backwards when you drive it up steep hills.
It makes no difference to power, so long as you let the engine rev up to 3,000 or 3,500 rpm without the mower travelling so fast it leaves you behind and goes away on its own. It might never find its way home.
