The original carburetor seems to be a Mikuni vm26 and it is widely used on china-atv:s and dual sports as different copy designs. It should be good for a 200cc, sometimes even 250 on ATV:s so my first recommendation would be to rejet it rather than try to get a bigger one, as jets don't cost much at all. If that fails to deliver I'd look for a carb with similar fixing points but a bigger throat like a 32mm but probly not bigger, keeping in mind that with a two stroke a carb alone won't do much you'll also be needing an exhaust with a power chamber.
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