Shiftkit topspeed?????

81 kph is about 50 or 51mph which is nice and fast even for a jack shafted setup. I'm assuming you have done some port work and raised your compression some to get enough power to go that fast... typically a stock China girl engine can push a bike up to around 40mph by changing the gearing but at a cost of climbing capability and they take forever to accelerate when geared that high... and the wind resistance is what keeps a stock engine from going any faster so you did good with whatever you did to your engine. If it took a long time to hit that speed you might be able to hit the same speed or gain a mph or 2 by switching to the 10 tooth on the shafts or going with a 15 tooth on the cassette if possible, it may give the same top speed with a little better acceleration or maybe another mph or 2 if wind resistance is what's holding it back, but 51mph or 81kph is still impressive.