Try bringing your car to another shop... from what area are you? Maybe the other peeps can suggest good/reputable shops near your place.

On the carb, upgrading it or changing it to a twinside might cause some problems... i'm not so familiar with carbs but the twin carbs may just drown your engine given its small displacement. I'd suggest you have your carb overhauled and cleaned though... sa Kamias/Kamuning maraming carb specialty shops and its not that expensive. They can also tune and adjust your carb to its proper specs there.