My diagnosis would be that the rings are done for. This sounds like a ton of blow by past the rings which is pressurizing the bottom of the engine. This will make it smoke, blow oil all over the place, and be a real mess.
If you are lucky, its just the crankcase breather all stopped up with goo. I had an Ariel Square 4 and some bonehead screwed a bolt into the breather. This caused oil to be blown up past the rings and made it smoke like my 2 stroke race bike.
The easy solution? Just sell the guy a mosquito control officer's liscense.
