the key here Alward that I picked up was Sweden and winter = lean unless jetted properly for winter. fouled plug, reed, I don't think it would have quit, would have been hard to start for sure if at all. I would pull the pipe and look in the exhaust at the piston to see if it was smeared, could change the plug and try and hope that was it. the newer model KX 250 weren't like the old ones that came way to rich, later models were closer.
Boon, if it idled before it stopped it is fine, did you jet it richer? if the person you bought it from hasn't ridden it since summer, it was jetted for summer temps. You buy the bike, and now its winter and you decide to go for a ride and the temp is probably at least 50 degrees cooler than summer, so off you go down the rode and it quits. were you going slow or fast? if fast you shouldnt have fouled a plug. need more information