Personally I cannot think a reason to replace the carburetor, the 27mm carburetor on the Keeway TXM200 works good.
If the idea is to get more performance, you maybe want to replace the foam in the airbox, so the carburetor can breath less restricted – of course a less restricted exhaust system will also help performance.
Can anyone recommend a replacement carburetor? The stock one isn't very good, not easily adjustable. Is there a Keihin that will fit?