the gearbox filler hole on the spare minarelli 125 i have
,has what looks like a plastic breather tower onit,addmittedly it's pretty blocked up,has yours a 60mm tower with an short appendage angling back down from the top,think it's supposed to be hollow,for a breather application??,another quick test,run the motor,stick your thumb over the gearbox filler hole to see what sorta pressures there,if it's sorta pulsing fair bit,your engines crank seal might be sus allowing crankcase pressure to pressurise it's gearbox,justa thought,