How do you derestrict a derbi senda R 50cc and is it advisable?

When you do the exhaust, it means there's the potential for more exhaust fumes to leave the engine, but to keep the engine 'balanced' you therefore have to increase the amount of air (by an air filter) AND fuel (by a bigger jet in the carb), which will result in a more powerful engine. The sprocket can then be changed to alter the gearing to improve either acceleration or top speed, at the loss of the other.

mate