There already was a small diameter breather tube on the oiltank, that
has now been removed. Aad leaded the area around the welds,
and then we pressure tested and thoroughly cleaned the tank.
The magnet in the drainplug did it's job well, this was all
just from the drilling and welding, it did not come from the
engine oil! We dried the tank and refilled it with oil, then
started the bike. No more oil blowing out, on to the next issue,
the leaking primary cover....