I flew the following route in a light single in 2009 (with others):
Ulaan Baatar
Chita
Chulman
Yakutsk
Magadan
Anadyr
Nome
We had to truck in our own avgas from Kazakhstan apart from Anadyr where there was a cache of drums that had been under a snowdrift for half a decade or so.
There was petrol of a sort in the big places (eg Yakutsk) .... apparently ok in AN-2 but I would not trust it in my aircraft and I have used mogas on occasion.