Kawasaki kx250f seems to be overheating sat idle (thats if it sits idle - generally stalls if no revs when starting). Coolant leaks from overflow. Have checked oil and it seems used but doesnt really look creamy (but is definately not new oil by the look of it). Any ideas - I think the head gasket - but could it be anything else?

The big question is it overheating because of coolant loss (a leak or leaks) or is the overheating caused by a jammed thermostat, blocked radiator or the cooling fan not working.
This is the confusing chicken-and-egg situation that often faces the diagnostician, especially as when checked the coolant level is likely to be low in both cases and overheating can create coolant leaks...
Furthermore when an older engine overheats a lot or boils it can begin a chain of events that causes great expense and inconvenience over an extended period of time...

In your case it would seem the coolant is low because of a leak and that is what caused the overheating. Clearly you need to trace the leak and I can be of little help in the matter other than to suggest a hose can fail at any time and although regular inspections can greatly reduce the chances it can't entirely eliminate the possibility. Water pumps on the other hand, often aren't accessible for inspection without removing lots of stuff but they are generally very reliable and tend not to suddenly start gushing coolant. Most often they are responsible for a minor mystery coolant loss that gradually gets worse over an extended period.

I am wondering if the previous owner could have known something about the problem.

Locating the leak would generally mean getting under the car and removing any undershield components in order to narrow the area of the leak and then remove any further components until eyes can be rested on the actual source of the leak.

You have damaged the engine, the smoke is from burning coolant in the engine due to a blown leaking cylinder head gasket or gaskets. Do not drive this or you will completely destroy the engine.

REPLACE THE THERMOSTAT AND RADIATOR PRESSURE CAP.IF COOLANT LOW, ADD MORE COOLANT THROUGH COOLANT OVERFLOW UNTIL COOLANT LEVEL IN THE COOLANT OVERFLOW STAY AT FULL COLD MARK.REASON POUR COOLANT THROUGH COOLANT OVERFLOW MOST GM CARS DONT HAVE RADIATOR CAP.WHEN YOU GET COOLANT LEVEL OKAY, CRANK CAR LET IT IDLE UNTIL ENGINE GET HOT BUILD UP PRESSURE THEN TURN OFF CAR.CHECK FOR COOLANTS LEAKS AT THE RADIATOR HOSES,CHECK FOR LEAKS AT THE RADIATOR, CHECK FOR LEAKS AT WATER PUMP WEEP HOLE IF YOU SEE COOLANT LEAKING FROM WEEP HOLE REPLACE WATER PUMP. IF CAR OVERHEATING WHILE IDLING YOUR COOLING FANS NOT WORKING CHECK COOLING FANS FUSE AND RELAYS.IF FUSE AND RELAYS OKAY THE ENGINECOOLANT TEMPERATURE SENSOR BAD.

CAR OVERHEATING FIRST CHECK FOR LOW COOLANT IN THE RADIATOR COOLANT OVERFLOW JUG.IF COOLANT LEVEL IN OVERFLOW JUG LOW REFILL ADD COOLANT BACK TO THE FULL COLD MARK.PUT RADAITOR CAP ON COOLANT OVERFLOW JUG START ENGINE LET IT IDLE UNTIL IT OPERATING TEMPERATURE.MAKE SURE TOP RADIATOR HOSE GETTING HOT WHILE ENGINE WARM UP.IF TOP RADIATOR HOSE DONT GET HOT WHILE ENGINE IDLING YOUR THERMOSTAT COULD BE STUCK CLOSE CAUSING CAR TO OVERHEAT.IF TOP HOSE IS GETTING HOT.CHECK FOR COOLANT LEAKS CHECK TOP RADIATOR HOSE AND BOTTOM RADIATOR HOSE FOR LEAKS.CHECK FOR LEAKS AT THE RADIATOR CORES AND PLASTIC SIDE CONTAINERS.IF ALL IS GOOD ENGINE OVERHEATING WHILE IN A LONG TRAFFIC LINE, COOLANT FANS NOT COMING ON.COULD HAVE FAULTY ENGINE COOLANT TEMPERATURE SENSOR OR BLOWN COOLING FAN FUSE OR RELAY.COULD HAVE PCM FAULT.MAKE COOLING FAN ELECTRICAL CONNECTORS AND WIRING NOT DAMAGE OR BROKEN.

Sorry, could be a clogged up carb from sitting or a bad ignition box. Have you got a pal with the same bike you could change out the carb to try? If the bike sat for quite a while, it almost certainly needs a carb cleaning and service.

check under the car after it has sat for leaks if none found check you engine oil see if its milkey in colour also do you smell antfreeze as you drive? you might want to get it pressue tested for leaks

If it's overflowing.... I doubt it was just sitting for two hours. Overflowing is because of overheating or a blockage, which only affect the car when it's running. It is not overflowing, but simply leaking, then you have a leak. Pressure test the system, replace what's leaking, and, depending on the coolant condition, top up or flush it.

I suspect this vehicle has been overheated, because you asked about the lights for engine/coolant temperature, and because you said "overflowing". If this is the case, do the same thing as if it were a leak, but have a technician check the cylinder head for fractures and warping as well.

Next time pay attention to the temperature gauge and don't wait for the "idiot light" to come on on the dash. A slightly high gauge is a problem, but an engine/coolant temperature light is usually an expensive problem.

Check the intake manifold/s, even a small leak will make it run poor. Being stored for five years then putting it into use, you may have a blocked main jet. Sea-foam is a great product and I would have used that product to store the bike but not to get it running. It is slow acting so it doesnt work like carb cleaner.