tend to agree with tom there, a 250 in a lighter 150cc chassis is a cool prospect but time and $ are spent on fab and motor. the out come will be better than a stock 150 and average speed could range from 45-50mph depending on weight and gearing. but! other than carb , exhaust, clutching, electronics and sprocket gearing.....it is what it is. quite frankly if different gear sets for the final drive tranny were available like the gy6 engines, that alone would be a huge upgrade cuz the stock ratio sucks.
like tom stated no crank, head or cams and if you do get lucky you might find the discontinued BBK. and i have one installed on the CROSSHAMMER build and couldnt tell you it was worth the $.
my 155cc yerf has beaten to stock 250s in a drag race and out climbed a bunch of buggies with over twice the displacement, granted that one smoked a belt and another broke the bead on a tire but still the lil 155 pulled it upa hill it shouldn't have.
there have been plenty of times where a well tuned hot rodded gy6 has out classed a 250.
so it comes down to sink the $ into the gy6 or sink the $ in a 250, a rad, a carb, electronics, material and a good chunk of time.