Did the smoking and spewing oil begin suddenly or did it happen gradually? If it isn't a stuck breather valve, it sounds like an abnormal amount of blow-by, which gets into the crankcase past the rings or through the piston, and is more than the breather valve can cope with. How does the compression feel, when you pull the starter? Different from a four-stroke rotary mower?
If the problem developed over a period of months or years, it sounds like you need new piston rings. If it happened suddenly, something has broken somewhere.
