Just do the pro-gamer move and define the economy in real goods production terms. Soviet economic year books were just like that, preferring hard reliable data of goods produced over GNP figures. If you know Russian, check up istmat http://istmat.info/
It has all kinds of documents from those times, including sources from Imperial Russia, even. It's revisionist-dominated, though, due to khruschevites rewriting history before them, so post-1956 dated material has vastly different data than pre-1956 data, and on top of that pre-1956 data is EXTREMELY hard to find. Hell, references to it in anti-communist literature are easier to find than the things they claim to debunk.
I don't know how you will make a game with data like this, though. Present day society will refuse to believe this stuff. If I was going to do a game like this, I'd make a WW2 strategy game with stalinist data, like 4 million dead fascists in 1941, and call it "alternate history", while in game itself it will explain the real world events way more accurate than the "real" history. Say, a cutscene with real life Japanese attache overseeing the bloodbath Soviets are inflicting on the Germany. reporting it to his higher-ups, and they deciding to remove troops from the Soviet border, allowing USSR to move fresh siberian divisions West for 1941-42 campaigns.