Re: Which engine?
None of the chinagirls are industrial/commercial-rated engines. None of them. They aren't designed to run all day. See what happens when you run them at their power peak for a an hour or two straight. See what happens when you do that every day. I have, and I was constantly wrenching! Some people get/build a gem of a 2-stroke, but most don't. In the end, it's still an engine that wasn't designed to run all day.
OTOH just about every 4-stroke is. HS, GX, HF, Predator are all industrial/commercial engines. Most of them have their own cooling fan, iron cylinder liner, and pretty much guaranteed to run all day long at their power peak. And there's always the not fiddling around with oil all the time bonus, too.
My years of experience with these have led me to this opinion: if one is intending to build a cheap toy to have fun with now and then, a chinagirl is great for that. But if you intend on relying on your motorized bicycle to get you to important places, then you should go 4-stroke. It costs more now, but less later.
"If you can't afford to do it right the first time, you must afford to do it twice."