Several options. I have been running the Keihin 38mm PWK carb and like it. I have had lots of these on my dirt bikes and other bikes and they are easy peasy.
Mikuni has its version and Im sure many others will chime in with what they use.
They are for sale all over and I threw a couple on my website as well.
I forget what the carb side of the throttle cable looks like, but it may need to be altered or replaced as well.
https://dcmotive.com/products/keihin-38mm-pwk38-carburetor