Peculiar & cheap engine (kits) on chinese websites

I would say that the first kit you listed is for a pocket bike. I buy bike parts & etc on Aliexpress all the time, but it is very important to be aware of the fact that some vendors aren't selling at wholesale prices. What they do is buy locally at wholesale or even retail and then add their cut on top which makes the price more expensive for the same thing.
The other trap is that some vendors will seem to be offering free shipping when they aren't at all and if you buy from them you'll quickly receive a message to say that there are additional shipping charges. For this reason it's very important to ask about shipping charges first.
I'm not saying it's all bad. I've made purchases from Chinese traders who have offered the very best in customer service as well as some really great deals. When I find a trader like that I make sure to buy from them again in the future in order to avoid disappointments and having to resort to Aliexpress's dispute procedures.