I think it is a blown head gasket - could this be correct?

Hi,
Well, it is possible though rare these days if it is fairly new. Blown next to the pushrods (OHV engine) would pressurize the crankcase.
Could also be a fuel problem that has dumped gas into the oil; oil level would be high and it would smeel of gas on the dipstick.
A clogged air filter would result in blackish smoke, unless it is clogged by oil saturation.
My experience is that the gas in the oil is the most common problem these days because of the quality of fuel.
Peace,
Clarke