IIRC stock for a CBR F3 is 14/43. I know you can get a 13T front and 46T rear in the aftermarket but I don't think you will get a 70 tooth even going custom.
Your best bet would be to try to find a place that sells stunt bike parts. Typically that niche market isn't concerned about speed but power to do their tricks...Which is promising for what you need.
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