Welcome-That's a pretty heavy Silverfox!!! At least you have an engine that'll make some torque. How about some pic's.
What size driven unit do you have now? I'd guess with a standard 7-1/2" driven and your 22" tires you'd need around a 40t-46t axle sprocket to reach 40-45mph. There are plenty of ratio calculators on-line available for you to input in your components and see which way to go.
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