Aye Fatboy,
Suspect the fuel tap attached to the left hand frame tube had been left in the 'Prime' position or else the diaphram in the tap isn't working and preventing the flow of petrol to the carburetter float bowl. In the carburetter float bowl some dirt has stuck between the float needle and it's seat, therebye allowing the petrol to run down the inlet tract, past the valve, down the the side of the piston past the rings and fill up the sump.
Others may have different opinions and consider their points as well as mine, I'm fallible!

It would appear that you need to do a thorough clean out and check of the fuel side of things and also the oil side. Get some 2/ oil in the bore afore you go spinning the motor over, just to give some lubrication to the rings.
I hope that it all sorts out OK.
My regards, Bill.