You could pay the Earth for a slightly longer 'Lambretta specific' cable assembly or use an extended top hat to fit the crankcase. That's the usual cure by looping the cable over or under the obstruction.
Or, you could acquire an even longer car handbrake cable & route under the crankcase before curving up to the lever & operate that via the 'reverse pull' principle. Having often utilised old car handbrake cables, they are always nylon lined & usually have a long M6 threaded portion on one end. I've never had to resort to looping under but have seen the method on sidecar racers so know it works well in quickly retarding high speed, heavy three wheelers two-up
