Gearbox leak on Model 50 - any suggestions.

You used to be able to get a conversion done to the outer cover, to replace the o rings with proper lip seals, Hemmings did it I think and Norvil too, but they no longer advertise it.

RGM and possibly others sell (or used to sell) X-rings of the correct size for the main two gearbox seals.  One has been in my Dommie for some years and the kickstart oil leak has been cured - at least for the few thousand miles it's done.  The gear change has two coaxial leak paths - one for the lever (which moves backwards and forwards over a constant range) and one for the gear indicator inside it.  As far as I remember, the X ring they sell only deals with the main lever, but it seems to be effective (if not perfect).

On the kickstart, the depth of the groove is too big on mine. I don't know if previous owner milled it out to replace the bush.  But I had to fill the back of the groove with silicone before putting the X-ring in and re-fitting the cover over the shaft before it set.  With sellotape to make sure the splines don't cut the seal. Don't know if that's normal or not.

I once tried in vain to  find a selection box for x-rings.

O-ring strip can be found.  You can make O rings surprisingly easily using it with a sharp scalpel and superglue, and good eyesight or magnifier.   

... are fairly readily available eg https://www.ashtonseals.co.uk/generalpurposeseals-x-rings

Re making your own O rings - that's how I first became aware of O rings as a technical apprentice back in the 1960s. I never even knew you could buy the. Of course this was for megawatt steam turbines.....

You could try fitting  a breather to the box and slightly reducing the oil level