Gotta increase the bore and the stroke of the motor. The bore would involve increasing the diameter of the cylinder and thus the piston. The storke would involve using a crank with a longer throw, as it is commonly called. But htere is more to it than just throwing another crank in there to do this. Making sure the crank doesn't hit the block anywhere at the bottom, the skirt, is very important. Also the deck height comes in to play. I forget what the exact places to measure deck height, but it involves the top of the cylinder block and the cylinder head. You don't want the piston traveling upward and hitting the cylinder head either.