The obvious comment is to drive it more or turn it over in a ventilated garage. As said it is oil leaking from the tank via the pump. Shut off valves work but if you forget them the engine is rapidly ruined.
On British bikes refurbing the worn pump would help given their age & wear.
I don't know enough about S&S engines to comment on the design.
You could also pull the plug leads and crank the engine to scavenge the oil before starting the engine.
