Missiles not so much, all the vehicles are different story. Serbia does not have any capabilities to transport launchers. No civilian plane can transport it and
AFAIK no Russian plane can come to Serbia. And you really think Serbia would risk it's future aspirations of joining EU to send a dozen of missiles to Russia?
If we even consider that China wants to sell FK3 to Russia, if you look at the map, it would be a lot easier for them to deliver it to Irkutsk via air/roads then to go this route. Not to mention that everyone knows that Serbia bought it and got it delivered and NATO does have it's office in Belgrade.
So, still waiting on explanation how these systems will get delivered to Russia?
And let's not forget that Serbia is negotiating to buy 12 Rafales from France. If anything, to me it seems Serbia is slowly distancing itself from Russia.