Dirk the best cure for this problem is to fit a modern double lipped sealed bearing in the gearbox case. But remove the inner seal with a small screwdriver. This of course will entail removal and total strip of the box. Maybe Darren Wrudd from this forum can help you with the bearing. Several of us have done this modification and I just took my original bearing to my local bearing shop and they got one for me.
Yours is excessive though which sounds like your sleeve gear bush is badly worn, so definitely in need of an overhaul.
If you don't feel capable of such an overhaul? Maybe contact Peter Long in Germany and ask his advice about a possible engineer. http://www.cornucopia-enterprises.de/