Flight in Russian airspace, with a VERY few exceptions, requires the services, on board, of a Russian assigned "navigator". You have to pay him or her, as well as provide food, lodging, etc.
Also, you don't "fly around" in Russian airspace. They have very specifically designated airways that you may use to go from point to point, but you are not permitted to "fly around" there.
There is very little, if any aviation fuel available in Russia, or for that matter, anywhere else in the world except the USA.
It is possible to fly THROUGH Russia, but it takes a LOT of pre-planning, lots of permits, and the aforementioned "navigator".
Note a few years ago, a fellow flew his Kitfox from South Africa to Oshkosh, via Russia. In a Kitfox, with extra tankage, he still had to carry a "navigator". The guy was mostly useful for pouring gas from the one gallon jugs into the ferry tank, which was "installed" on his lap, according to the pilot.
Don't know anything about China, but I'm betting they make Russia look wide open as far as aviation goes.
And, there won't be any avgas there either.
Do you trust Chinese autogas?
MTV