I've changed the cogs on my TE450 and can say that it does not make a good dualsport. 17t in the front, taking off in first gear must use revs to keep from stalling. 55mph still revs the motor, it will do over 100 with the gearing!
Plus you are going to need to extend the fuel range. Find a different bike or if you could change the trans ratios that might work.