Stock motors suck tbh. To get the engine running good, slight porting (and crank balancing, crankcase and cylinder matching, exhaust and intake modifications, setting up the carb precisely etc) helps a lot if done correctly. And if you are after (a lot) more power then there is hundreds of other things to do than porting as well.
But one thing is sure, if you ask what porting is, what it gives you and how its done, then you definitely are not the man for the job (yet).
I've noticed that some of these chinese engines are of very poor quality (explains why they're so cheap). Some tuners are using pistons other than the original part, sleeved the cylinders and so on, all that to get the long lasting high output engine.