Well, gents, I believe I figured out my problem: I happened to notice some very fine droplets of motor oil on the headfins on the shaft side of my engine, and on top of the stainless-steel gearbox cover that's adjacent to those fins. Apparently, the Chinese use cylinder head casting techniques that result in thin spots and other problems.
My wife has high-powered dental loupes with a very bright LED light array mounted between the lenses so that she can clearly see inside her patients' mouths. I borrowed the loupes and took a look at my Huasheng, and I clearly saw very fine pinholes in the metal between the headfins.
These pinholes would explain why my engine would run somewhat OK until it was hot: the hot metal would expand, making the holes larger, sucking in air (and spattering out a bit of Mobil 1) and causing the engine to bog.