As far as I know, that sputter happens on all those generic engines.
I had a very bad experience with my generic 49cc scooter. Many parts boke. About five repairs in (muffler cracking and falling off, disk brakes locking, screws unscrewing from the vibrations, to just name a few of the problems), the engine casing boke. It couldn't hold on the scooter frame anymore. The funny thing is, the scooter is labeled off-road. It broke on road...
Anyways, today I picked up a HF 79cc (great 25% off coupon today). Yes, I am rebuilding it completely. I will be building it off of the same frame, as it is pretty good, and keeping the wheels (not tires).