A two cycle engine has a powerstroke every 2nd cycle of the motor
A four cycle engine has a powerstroke every 4th cycle of the motor
So given an identical tune 50cc two stroke and a 50cc four stroke you get 50% more power from the two stroke than the four. Its not as simple as that as you get into fun stuff like Schneurle loop porting and things of that nature..
Basically due to the nature of the two stroke - its easier to have - in small engines at least - a mixture containing both lubricant and petrol (known as petroil) - which means its a total loss oiling system - in reality the modern engine burns very cleanly and what it doesnt burn can be cleaned by a catalytic convertor. Villiers and other makers designed engines way back when with an autolube system that lubricated the motor much as a four stroke would be - and allowed a separate tank for petrol and oil - a more elegant but more complex solution. Complex equals money so we are left with the petroil engines we have now.
Because of the design of two strokes it is possible to get more power out of an engine by using a tuned pipe - in some cases up to 40% increases, which cannot be done with four cycles because they generally are valved engines where two strokes either arent or use rotary port valving systems and they use an entirely different intake/output methodology.
So to recap:
- upto 140% the power of the equivalent swept volume 4 stroke
- very tractable to tuning with tuned exhaust pipes
- Petroil type motors require a mixture of petrol and oil
- older engines polluting, new ones not so much
- simplicity - at a minimum three moving parts
- less power for a given volume
- tuning on small engines is complicated and requires expensive tooling
- use a reservoir of oil within the motor - either splash, mist or pressure lubrication
- less polluting than petroil two strokes - autolube machines there are no results for
-more complex - valve gear/cams and ancilliaries needed - many more moving parts