Aut cum scuto aut in scuto
It's Russian; it'll probably still fire. Say what you will about Russian equipment, (and I've said plenty of nasty things about it over the years), but it was made to be issued to the illiterate peasants who constituted about 80% of the Soviet population. As such, most of their weapons are extremely durable, if often quite unrefined in construction. I've seen AKs in such bad condition that they logically shouldn't even be in one piece but that would fire reliably.
This guy could probably save it:https://www.youtube.com/watch?v=9uFTbReFsGkAnd this lady would shoot it:https://www.youtube.com/watch?v=BPH7m_3sZNwAnd yes, Russian firearms are designed to be maintained in the field by armorers with a scrap of pig iron and a bastard file.
Of course it's repairable:http://www.commanderzero.com/?p=6513
There is a series about restoring a ditch find 1911 that is great - https://m.youtube.com/watch?v=B28_VN8Q6YE
Is it caked in mud, or is the rust so bad it's growing and flaking off of it? Bad either way.
I remember an article long ago about how the Russian gangsters were hitting the old battlefields where they dug up the ancient sidearms and MG of the dead and all of them, German and Russian, were repairable and deadly to those downrange.