Engine downsizing can effectively improve the fuel economy of spark ignition (SI) gasoline engines, but extreme downsizing is limited by knocking combustion and low-speed pre-ignition at higher loads. A 2-stroke SI engine can produce higher upper load compared to its naturally aspirated 4-stroke counterpart with the same displacement due to the double firing frequency at the same engine speed. To determine the potential of a downsized two-cylinder 2-stroke poppet valve SI gasoline engine with 0.7 L displacement in place of a naturally aspirated 1.6 L gasoline (NA4SG) engine, one-dimensional models for the 2-stroke gasoline engine with a single turbocharger and a two-stage supercharger-turbocharger boosting system were set up and validated by experimental results. The simulation results show that when a single-stage turbocharger with wastegate is used in the two-cylinder 2-stroke poppet valve gasoline engine, a compressor with high pressure ratio at low mass flow rate should be selected in order to maintain the positive pressure difference between intake and exhaust ports. The maximum brake torque of the turbocharged 2-stroke gasoline engine cannot reach that of the NA4SG engine at 1000 rpm and 1500 rpm. Its maximum brake power at 3000 rpm is lower than that of the NA4SG engine at 6000 rpm. However, in the case of two-stage serial boosting system with a supercharger and a downstream turbocharger, the 2-stroke poppet valve gasoline engine can produce the torque and power of the NA4SG engine.