While a lot of the newer sleds have gone to four stroke engines, there are still an lot of fuel injected two strokes in the newer snowmobiles as well. Ski-Doo offers the "E-TEC" direct injected two stroke that is actually cleaner emissions wise than a comparable four stroke. Yamaha is nearly all four stroke now, while the other OEM's offer a mix of two and four stroke engines.
For the type of environment a snowmobile operates under, it's tough to beat the simplicity and light weight of a two stroke engine. Besides that, a well tuned two stroke will beat the brains out of a four stroke of similar displacement, all other factors being equal. That's why some of the newer four stroke snowmobiles are offered with turbo's and superchargers to get the power where it needs to be, but they still suffer from weight and complexity issues.
NHRA only runs the Pro Stock snowmobiles, but the Outlaw class of sleds are running turbo four strokes putting out over 600 hp, as well as BIG two stroke engines on spray. These snowmobiles were running in the low 7's at over 180 mph on the asphalt quarter mile, but were pulled back to 1000' earlier this year due to safety concerns. Now they are in the upper 6's at around 170 mph in 1000'.
Sorry for getting carried away here. I know this is a car and bike NHRA Forum, but being involved with the sleds as I am, I get pretty enthused when it comes to discussing the tech aspects of what I believe to be very well engineered and exciting drag racing vehicles.