Well brother if you are planning to install cng kit,you will have a decrease in performance as compared to petrol operating engine.To keep the performance at its best,you have to use platinum or iridium spark plug.Iridium is expensive,but it will pay in the long run.Performance as well as life are the main benefits of iridium.The recommended plugs are as follows.
1.AUTOLITE xtreme performance iridium@Rs.850 per piece
2.AUTOLITE platinum@Rs.300 per piece
You can get them from nadir autos,gawalmundi,rawalpindi