Cutting the crankshaft counterweight like that will induce several negative effects.
-Promoting of turbulence
-Unbalancing the rotating assembly
-Increase in crank case volume
-Interruption of airflow

Those things decrease performance individually and do the same when paired together.

The more air and fuel you can move through the engine without blowby the more power it will make. Velocity is a key thing these engines use to make power. Efficient airflow is also something that will make power. While it is not possible to fully balance a single cylinder 2 stroke engine there is a tolerance to get it close and within specs. Cutting the counter balance like that removes all of that and throws it out the window. This will result in alot of physical and sonic vibration in the engine quickly prematurely wearing out the engine bearings, crank pin, rod bushings, wristpin and eventually the piston and sleeve. Crankcase Volume is usually close to optimum is most engines out there. When you increase that volume you decrease flow and velocity which will take away power.