Why don't you gear it? Stock, you're 18/39, which nets you a 2.16 gear ratio.
Raising the upper sprocket, or lowering the lower sprocket would net you a better ratio.
16/33: 2.062
16/35: 2.187
16/39: 2.437
17/33: 1.941
17/35: 2.058
17/39: 2.294
18/33: 1.833
18/35: 1.944
18/39: 2.166
19/33: 1.736
19/35: 1.842
19/39: 2.052
Upper Sprocket
88F-17682-60-00 16tooth
88F-17682-70-00 17tooth
88F-17682-80-00 18tooth
88F-17682-90-00 19tooth
Lower Sprocket
88F-47587-00-00 33 tooth
88F-47587-20-00 35 tooth
88F-47587-90-00 39 tooth
I couldn't find the part numbers for any other lower sprockets.
Use this calculator to determine what your new top speed would be with different gearing,
http://www.turboaddictionparts.com/catalog/snowmobilecalculator.html
I calculated, with 18/39 gearing, with an RPM of 7500, 9 tooth drivers(2.52pitch), your top MPH is around 74MPH?
Change the lower gear to 18/35 and your top MPH changes to 82MPH, thats a gain of 6Mph!
Change the upper to a 19/35 and your now at 87MPH! For a gain of 13MPH!
Or go all out! Change to a 19/33 and your top speed is theorheticaly 98MPH!
Now, this is all given that your engine can push itself to those speeds with enough HP...