Gearing fo' sho'. What do you have on there now?
They are fairly heavy bikes. You could try opening the intake and exhaust ports..I have not seen the Zen pipe headers up close, so I'm just guessing that it would need to be matched to the cylinder.
38 is pretty good for that bike with only a new exhaust and upjetting. With the gearing, you will have to experiment as you will either lose top end, or lose bottom end depending on what you're going for.