Crankcase stuffers are a benefit. I fitted them, one per crankcase on the inner side to my engine quite a few years back. You have to be careful they don’t impede the gas flow from the inlet ports and into the transfer port. Mine are 25cc each and carefully shaped. They don’t reduce the volume of charge, they increase it. It’s like using a tyre pump (the piston and cylinder) on a bicycle tyre (small crankcase volume) and a car tyre (larger crankcase volume). One stroke of the pump has more effect on the bicycle tyre than the car tyre and then when the piston compresses the charge in the crankcase with a smaller volume it pumps the charge at a higher pressure and bigger flow into the cylinder. Fillng the outer part of the inlet annulus where there are no inlet ports also has the same effect so together with stuffer blocks makes a significant improvement.
The stuffer blocks have another advantage, they cause turbulance inside the piston helping to cool the crown and also increase little end lubrication.