I'm getting terrible mileage on my Xingyue 150 cc and recently lost power (maxs at less than 40mph). Any ideas?

108.5Nm@5000rpm Key Specifications of Chevrolet Sail
ARAI Mileage 18.2 kmpl Engine Displacement (cc) No. of cylinder Max Power (bhp@rpm) 82.4bhp@6000rpm Max Torque (nm@rpm) 108.5Nm@5000rpm

could be a faulty power control swotch
I am inclined that the voltage regulator the controls the amp out put is faulty
Take it in for a specialist diagnosis to a welder repair shop

Check coolant temp sensor. Is tailpipe black?

may need a tune up ie. plugs/air filter/ fuel filter any on these things can cause poor fuel economy but are not limited to just this. this is a good staring point to help improve the mileage. hope this helps

http://en.wikipedia.org/wiki/Bajaj_Pulsar is a good source of information on that bike.
or here http://indiareviews.wordpress.com/2006/08/17/p-180-dtsi-long-term-review/

You will need brakets from a F-100/150 73-79 2wd 302 engine then the windsor will bolt up.

carb needs to be cleaned and rebuilt

If I were you I would stay with the automatic because a 5 speed will actually use more gas in the long run and cost you less than replacing everything to a 5 spd.. Try playing with the timing and carburetor to get the max mpg.