The engine is the core part of the motorcycle, the performance of the engine determines the performance of the motorcycle, the engine power should be suitable to your motorcycles or scooters. now engine can be divide into 4 stroke and 2 stroke. Most big power motorcycles  use 4 stroke engines. And 4 stroke engine is  more good to our environment. Before you choosing engines, you should also check the assembly dimension ,also need check it is for scooter, or for motorcycle or for ATV. They are totally  different types of engines.