Somehow moisture is getting into the cylinder. That wrecks the spark. I'm convinced of it. Whenever we dry them out with dry heat, these engines will start and run fine. I've tried every gasoline additive that exists. Nothing works. Dry heat applied to the cylinder does work. It might as well be a heat lamp, an outdoor flood light, or a hair dryer. Take your pick.
Have used a 200w magnetic heater on the exhaust manifold (as it's the only magnetic thing on the engine other than the oil filter) for at least an hour or 2. But my Sportsman is also 20 yrs old now, has over 5200 miles on it now so I wouldn't be surprised if the engine is getting a little tired too. I got it in 2009 from a Polaris shop that got it from the 2nd owner who just used it for mainly for trail riding. Just going to run it until something major breaks on it or I can't get any parts at all for it anymore.