He passed away in 1973, so his works are still under copyright.
Maybe in this case there is an exception? Mosolov's music hasn't been in print for many, many years, I would suspect, so there would be no publishers to piss off, in this case.
If UE have those pieces, they'll be able to sell you a copy - almost certainly an authorised photocopy, but a copy of some sort. Just get in touch and ask.