I researched 4-stroke snowmobile engines. 4-strokes are fairly new for snowmobiles, with gradually increasing acceptance. But there aren't many choices yet, and they are too heavy and powerful for my U/L. Unfortunately, all the 4-stroke snowmobile engines are water cooled, and are very high performance (expensive and complex).
Some of the snowmobile manufacturers have made or still make 4-stroke sleds, with the smallest engines in the 500 cc to 660 cc range, twins or triples. But these are fire breathing, fuel injected, dual overhead cam screamers, in the 48 to 80 Hp range, similar to sport motorcycle engines.
Arctic Cat had a 53 Hp, 3 cylinder engine, several years ago on a model called 4-stroke Touring. Polaris made the Frontier with a 48 Hp twin. Yamaha makes a 499cc, 80 Hp twin, called the Phazer.
They also have 1,000+ cc models with over 100 Hp.
Due to increasing EPA regulation, 4-stroke sleds are expected to become more common. So there should be more choices soon. One struggling, upstart company is supposed to come out with a 300 cc, 22 Hp, low tech single very soon.