Need to make it faster cheaply

Hi Guyz i just wanted to know some suggestion on my nissan.i need to make it faster,its very slow and im a racer so i lose races even with 1000 cc cars.i need to make it faster and alter the engine from the inside so that people could consider it a stock car unless they let it check from a mechanic.ITS A 16 VALVE TWIN CAM with carby.need no straight piping or other stuff which would probably make the people believe thats it an altered car.just some alteration in the engine like the pistons or whatever.i'll be very thankful if you suggest me some things quickly because i have a race in the coming days.THANKS