The documentary says that original expedition swapped a whole 600kg gearbox on a Kharkovchanka when one of them failed. Crew worked for 5 days at less than -50C.

So they ceranly did change much more than just belts :)

Do they even use belts? I imagine it's all internally chained, that way a diesel engine can last well over 100k miles... With oil change or at least top up, however. Mechanical pump/injectors don't even need electricity, just have fuel.