1. Single-speed engines made in Soviet Union are much more reliable, as modern China-made engines.
but
2. Youngest of those mopeds are made 20 years ago, and all these years rural youth had used them and "repaired" using a hammer. So very few of them are in a good shape. So, most of all, if you buy Soviet singlespeed engine - you have to repair it normally. "In ordinar condition", as sold in Internet - they are running, but "on the wind and the pray".