USSR version of the engine looks far more robust than the china girl copy.
Definitely.
is there some dusty old ware house somewhere in mother Russia with an entire stock pile of NOS motor bikes?
Possible, but not so much. Most of all engines and mopeds can be found in garages, basements, etc, people bought it.
When USSR was almost ending, there was economical situation - people had enough money, but nothing to buy. Deficit, you know. Housing was almost for free, food - very cheap, and sums of money were left.
So people often bought something, even if not needed, just to spend a money. Our family had big packs of washing powder, appr. 5-6 brand new mixers and hand drills, and so on.
The same with mopeds - some people bought them even without any intention to use, and these things just stayed there, before now. Absolutely sure - somewhere in garages something still can be found.
A year ago me by myself bought brand new (well, not used, sounds funny to call "new" things 50+ y.o.
) parts of old D4 engine kit:
Remained intact since bought in 1958...
But also warehouse can be found. For example - one person found warehouse with remainings of "Kroha" ("A little one" in translation) motorized bicycles:
and made some business out of it...
