I used a 16vm mikuni knock off for a while on a regular nc50 express. It has a cable choke and it uses the same mounting, i.e. the two bolt flange deal. Had to open the holes on the carb a litte, or on the intake, but only like 2 mils out, easy file or dremel work. Right no I am running my kitted express on a 19 phbg knock off I got from this dude
http://shop.hkmotos.com/product_info.php/products_id/69?osCsid=19adf54b06ce68b9a92f6e2b8edb90cc
I used the rubber intake mount thing that came with it and drilled 2 new holes in it to match the holes on the express intake. It works well for me, and the urban and regular express have the same intakes, but 19mm but be a little much on a stock bike. I guess if you didn't open up your intake at all, you would only be getting like 16mm worth of mixture, so that might be ok. Cheap experiment for sure though. It has a pull choke, so thats a non issue. Everything you need is in that package deal, except you are going to have to trim down your throttle cable housing maybe 2" or so.