These engines are a crapshoot. You might get a good one or a dud.
For those of you recommending RAW engines, I'd have to agree, except RAW doesn't make engines anymore!
Various factories make these bike engines, but most of them are identical. It's all about the [lack of] care they take in making them, and you could get one that is problematic or one that has no problems at all. The best thing to do with any engine, before installation, is to take it apart and clean it; inspect it for any shards of metal or unusual sharp edges in the cylinder. If you can, replace all the stock mounting hardware with quality studs and nuts.
It's a common misconception that these engines can just be slapped on a bike and it will run correctly indefinitely. While this will essentially work, your engine and the rest of the hardware won't last. Take the proper steps to prepping your engine and upgrading the peripherals(carb, fuel line, mounting, etc), and you will get something more reliable.