It's an air cooled engine, so head gasket is not likely to be the cause of white residue in the oil. Normally the residue is down to water getting into the oil. If you've done a lot of short journeys where the engine doesn't get up to temperature, condensation can build up in the engine producing the same affect. Being an air cooled engine, I also find it hard to believe it's overheated at this time of year. Maybe back in the summer, whilst sitting in traffic? I wouldn't be convinced it's overheated... :-?
