If you rebuild it to stock you should get similar HP and torque numbers that the engine had when new. If you were to bore the engine and put in oversized pistons, this would increase the actual size of the engine displacement and you should get slightly higher horsepower ratings. If you have a GTP you can increase HP by putting on a smaller pulley and opening up the exhaust by allowing the engine to breathe better. Opening up the exhaust will help the NA engine too but not to the extent of a supercharged engine.