Keep in mind that you need clearance between the sprocket and the case, some larger sprockets will require the case to be ground away for clearance. Or so I've heard. Just sold my only minarelli bike, I kept the stock 10 tooth front with a 44 or 45 in the back, did 35mph with 16 inch wheels on the stock lightly ported cylinder with a pipe. Keep in mind I'm 160lbs fully dressed.
With a cylinder kit you can afford taller gearing without losing much low end. But depending on swingarm clearance, it may make more sense to change the rear sprocket to a smaller one than the front to a larger one, to avoid grinding the case.
Seeing as you're replacing both sprockets anyway, I would check and see how much room you have, and report back. You'll probably want to go with somewhere around 10 x 42 gears, just a guess? Dont want to rev too high with a bushing crank if you have one. Keep it under 9k http://toolbox.martysgarage.info
Just my .2(edited)