Why would you want a quiet exhaust in Thailand; the police clap when I go past.
If you want to fit a different carb; which is certainly the way to go, then I would fit a pj/pwk.
In Thailand you have Tyga and they make a carb insulator for this type of carb - about 1200 baht I think. I also happen to have a
pwk39 which I bought second hand. They are about 10K new and if you wanted mine you could have it for what I paid for it - about 3.5K. I will probably have jets and needles in the next couple of months as well.
The 250 carb will certainly work well but where to get the adapter? Maybe somebody else knows?
I don't think tyga stock the exhausts anymore but Matt would be able to tell you if the one you have is good or not from the photo???
You could also fit the front master cylinder from the 250 as well. The 250's are really cheap over here - 30/40k. That would be like swapping everything.
The 150 cylinders can be ported considerably but if you want to ask Tyga; you will have to join the queue (I am next). I've just bought a book and a dremmel to give it a bash myself - what could go wrong.
Let me know if you want the carb; I am going to the Uk on Thursday

You are also welcome to the next 'nsr-world' meeting in Peppermint agogo

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mc18 F3, mc21 300, mc21ish sp, mc21/28ish.
cbr929, RC30, tdr250, rvf400, wave, gyro, joker.