Traditionally, an extra long cable of a small gauge, going down the RH underside of the legshields. Where it exits near the choke control, loop it over the top to the carb. Alternatively, an elbow similar to the choke cable elbow will work & be neat & compact & allow a carb inlet elbow & filter to be used.
Save yourself a packet by buying the stainless inner/outer from a cycle spares merchant. Tandem cables are superb
Similarly, any cable fittings are available from 'ped merchants for far less than 'Lambretta merchant' money
(I always use the gear change pulley for 30+ mm carbs & add a 'Serveta' type spring to the throttle rod to help self closing. It doesn't have to be a specific torsion spring on the rod as any tension spring that fits the rod snugly can be adapted to do the same job. One end is clamped via the rose joint clamps, the other end goes into the eye of a split pin fitted the spring length or so away to snap the pulley shut. The headset may need clearance for a bigger pulley but a stop can be created by adding an M5 screw & nyloc nut through the unused pulley cable hole)