A head gasket is possible, a worn float valve contributing to fuel thinning the oil more likely. The contaminated oil and oil filter will need to be changed out if that is the case and the problem corrected of course. An inline fuel valve is also good insurance if you use it to prevent a reoccurence. You will have to determine if the fuel solenoid is still good or not. A person can nip the end of the plunger to get it up and running if the solenoid has failed -for test purposes of course.

That was it, fuel leaked into the block causing the oil to thin out and pass through into the carb.
All fixed now, thank you.