I know that setup well. It orignally had a cable, and there was a lever assembly on the handlebar, and it has been removed. With the cable setup, there is no detent for the lever, so it won't work properly.
The good news is that carb is less desirable, and a 30mm Mikuni carb (with no need for a cable) is cheap as chips. Like this one:
http://www.ebay.com/itm/MIKUNI-VM26-...eaa257&vxp=mtr Less than 25 GBP to your door.