The strokes of an engine are Intake, Compression, Combustion and Exhaust.
The difference between the 2 stroke and 4 stroke engine is that in a 2 stroke vehicle only two strokes are completed in one revolution of the cylinder while in case of 4 stroke engine all the four strokes are completed in one revolution of the engine.
The result is that 4 strokes are better in fuel consumption because in case of 2 stroke of vehicle little amount of fuel remains unburnt and is thus wasted.
In addition to that a 2 stroke vehicle usually needs a mixture of oil to be burnt with the fuel.
I hope this is enough to tell you that a 2 stroke engine can't be changed into a 4 stroke one.
Soon I shall post a tpoic here which will being clear to you the whole difference including the working of a jet engine. Stay tuned to the Bikes Forum.