I doubt you guys apart from Flarry have gearbox issues. Just slipping clutches. Because these engines get swapped into so many different bikes the clutch cables can differ and run out of adjustment. The cable needs to be adjusted so that it starts to engage as soon as you start to release it so the clutch springs have a chance to apply full pressure. Also the oil you use will effect the clutch performance.
The 140's also make mince meat of the alloy kickstart levers getting around which are junk, so use a steel one. They must be kicked from TDC properly if you want the starter gear to last, do not go thrashing at it. Any engine with higher compression can strip stock starter gears if you are careless.