There are a few ways to extend the steertube for mounting different stems on the particular ENGWE ebike, I've tried a several, still trying to finalize the most secure way for myself.
I think the method Gearbasher used is fine, as long as the top cap keeps a decent amount of tension on the whole setup.
I've also tried the hidden same hidden stem extender, it is made of aluminum, the center bolt is aluminum, too.. easily stripped when you try to torque it down. Also it is more difficult to have proper tension for it to secure the headset without a top cap that's threaded.
I currently have this steertube extender installed, easily combined with different MTB stems, not the lightest setup, but with a top cap and long bolt to use existing star nut in the steertube: :
https://www.ebay.com/itm/154650501497
If I ever find an aluminum BMX high rise handlebar, I'll probably go with something similar to gearbasher's setup; I don't think it's anymore dangerous than other setups for my riding; that's just me.