I guess there'll be several answers besides tig and not any old welder.
A couple of Laverda guys spring to mind. Piet in Germany and Laverda Scozia in Scotland.
If its a non matching numbers bike, you may find a set of crankcases cost the same or less.
Donor parts are ideal from scrap cases to repair, if you can find them. Looks like there is metal removed from around the seal/bearing housing but i cant see how much.
Ive posted a pic showing a case with a slice already missing from around the seal housing, if its through or almost to the bearing, it requires a great deal of care to prevent distortion of the bearing housing.
In your favour, the outer land is not holding the oil back as per pic.
Find someone who's done it before to save yourself headaches down the road.
Rob