Top quality Lambretta rear hub shim from A F Rayspeed. Fit this shim onto the rear hub bearing before fitting the bearing into the casing, this will increase the room in the casing for the loose gears by 0.2mm, but it will mean you need to make the main gearbox shim 0.2mm thicker. 
Multiple shims can be used to ajust all the loose gear height to the desired height.
 
0.2mm REAR HUB BEARING SHIM TO WORK WITH CLOSE RATIO 5 SPEED AND CYCLONE GEARBOX STACK SPACING
 
0.5mm are also available on this website here: