If by rusty you mean the chrome plated metal tops that come on the BS34s, you can swap alloy tops onto them from other model's carbs. Here's a set I just finished up as back-ups for my '83. The tops came from XS400 carbs .....
They can also be scrounged from XS750 triple carbs. These often have an angled side on them .....
They can also be scrounged from XS750 triple carbs. These often have an angled side on them .....