A great snow tire is the Ohustu pro-wedge but it's a little tall, another great alternative is the goodyear rawhide terra II, they are 21/11/8 and look exactly like little tractor tires. I have them on an old 125m and they are unstoppable in mud and snow as far as sport machines are concerned.

I put these on my R when i want to ride a few mudholes and the differenece between them and the holeshots is day and night.

 

Tyre studs, that are used for ice racing make any tyre a good winter tyre. Its what i use on mine most of the time. I used to try chains for a while, on my R that was until I riped off my rear fenders. But while they were on powder, ice anything. There was nothing better!

 

for deep snow paddel masters are the way to go!