Have you tried adjusting the carb? I mean the fuel/air screw on it. And are you sure the compression in the cylinder is good? Anyway, installing a performance exhaust will be fine, but if you install a larger carb the engine will reach its best performance at higher RPMs, which means that you will have to open the throttle wider to feel the pickup and power. Carbs with smaller diameter make the engine more powerful at low and medium RPMs -- in some cases you can even start moving without opening the trottle at all, just engaging the clutch slowly. So, it's up to you to choose.