The microfiche shows that you have a 21 top 43 bottom gears I would check to make sure this is right. Then I would drop a tooth on top and see how that works. You may loose a little top speed, but i'm not sure what you are looking for. If you don't care about top speed you could drop two teeth on top or go to a 44 tooth bottom gear. Try to find some one with used gears so that you can experiment with it and find what you are looking for.
You should check the gearing that you have now but if the microfiche is correct I would think a 19 tooth top gear would be a good start. According to my calcutations with a 156" track and 19 tooth top and 43 tooth bottom gears you should still have a theoretical max speed of @83 - 88 mph. For the time you want to go fast.
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