Cronk. One straight forward way might be to change the track drive axle, Go to a 7 tooth sprocket from an 8 tooth. The dealer might be able to correct the speed on the dash. Others on here will know the details and if this is a valid option for your sled. I have heard the 600 Ace on my Skandic WT has the 7 tooth and the 900 Ace has the 8 tooth. I'm not an expert on this, so be sure to get conformation before you jump on this approach.
buy top gear and for a 600ace
it has a 21t top gear and the 900 has 24 t gear for a big change
or not as big of a change go to the 51t bottom gear from a 49t
504152927 Sprocket, 21 Teeth
504152971 Sprocket, 51 Teeth
the stock chain may be too short for the 51
and maybe too long for the 21
the 900 uses 106 link chain
IMO go with the 51t
top speed using the 21 will be almost 10mph slower than using the 51t
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