I wonder why you would want a plunger BSA to go faster, maybe you have a death wish. I have seen the conversion as shown here, a pair of Mikuni carbs were fitted on rubber sleeves sourced from a wrecker. If you really want more performance fit a 357 cam and larger valves and upsize your single carb to rocket specs. Twin carbs are more trouble than they are worth. If you go to single carb I guess your head will be scrap.