Hey. Replacing the stock cam with a performance camshaft will help. To gain more horsepower you should also make the airflow to the engine better. Replacing the stock sylinder with a big bore 150cc kit will sure make the engine more willing. The engine on a 08 Derbi senda baja model, is the same engine that you will find on a Yamaha TTR 125. So that is what you should search for. Hope it helps:)

First thing id do is replace the stock exhaust pipe for a perforance one can work wonders for some bikes