New carb for a 273...

99.9% of aftermarket carbs have a larger diameter air horn and wont fit your commando air cleaner. To rebuild the stock carb may be best plan. The stock single plane intake and choking exhaust manifolds limit the performance you'll get from a 273 anyway. Keep the 273 all stock and set your sights on a 360 with a lot of aftermarket stuff if you want to burn rubber.