One thing that can always be done is changing the gear ratio by either adding a larger sprocket to the gear box side, or even changing the gears internally, as i think they have a few different ratios available. I would call the gear box manufacturer to see what is available. Also, when you are looking at it, see if there is plenty of room for a bigger sprocket on the gear box. Since the street bike motors have 6 gears, you should be able to gear that sucker down really low, and start in 2nd for normal driving, and use 1st for only slow speed crawling.
Just to give you an idea, i have never hit 4th gear with our R1 Weller set up... and i have had the car up to 100 mph (or pretty close). Top of 2nd is about 65 to 75 mph on the track. So you should have plenty of room for gearing to make it crawl really slow, and still be capable of high speeds.
G-