Methinks you still need to go through the exercise of thinking about what the ideal gearing should be, and not just hope your existing tranny and sprockets will do the job (if your tranny will even handle a 9").
Think about these questions:
What do you consider fast? i.e. 0-60 times.
What do you want for top speed?
How far do you want to travel and at what speed?
What is the steepest incline you need to get up?
What is the frontal area and CD of the bike?
What will the bike weigh with all that?
All these questions are pretty much interrelated. My gut feeling is that a 9" is not the limiting factor on a bike, but there is no reason to speculate. Pick a motor and a battery pack and a controller and do the math, and play with the gearing, rinse and repeat till you find a combination that can deliver what you want, then see if it is in your budget. It is a lot of number crunching. Less talky, more mathy
