Any Manufacturer ID? Some are long gone now. Garwood was a popular brand. Best you can do is take off pump, disassemble, and see if seals have a number on them or take old one or measurements to a bearing place, and see if they have or can get one or two. OR, live with the leak, most old units leak some, and forget it!
If you have the big single cylinder, it is also the resivoir for oil. Usually a pipe plug on top, fill to it's parked level till it runs out. Maybe the cover you mention is correct, not sure. There has to be some air space left in the higher end of unit, it won't be completely full.
