Re: 23hp Vanguard oil leak
Along with the above suggestions, you may have blow-by/bad piston rings, cracked/hole in pistonhead, scored cylinderwall(s). Could be running TOO RICH - black gassy plugs = not good. Could be burning oil, but that often can leave white-ish deposits on plugs. Gas in oil/crankcase = BAD (back to bad rings), reduces lubrication.
THEN ... cracked/warped head or block ...
So, while ya got it apart ya might as well consider checking & being prepared to replace rings, cylinder sleeves, push rod BEARINGS - but maybe not push rods unless they're knocking.
Watch - it'll probably be sumthin simple & cheap!