It's spring loaded, there is suppose to be a knob near the speedo and a cable you pull up and twist to keep the choke open. Better to find a $25 22mm lever choke carb.
You can if the carb choke lever allows it....My X15 with 25mm carb from an ATC 185 didnt have too many problems...I removed the choke bracket from the 19 and added it to the 25mm with little technical difficulty plus even added a pusher spring to get it to nreturn instantly and it worked flawlessly...Why can't I keep the old choke hooked up to the new carb?