A 1920 POD? That's tricky- it would be easier with a pre-1905 one.
No Nazis, instead a successful putsch by the KPD. Germany is now a Soviet ally, so their main foe is Japan. Therefore, all the effort which goes into modernising the army IOTL goes into building a Pacific Fleet which can re-fight Tsushima and win. The ships are built at a new yard in Vladivostok, with a huge drydock complex built by convict labour. It's a fleet built specifically for combat in the Sea of Japan, in range of land-based air power, where the IJN's carriers are less important. Therefore, in keeping with OTL later Soviet doctrine, it's built as a battlefleet rather than for power projection, with the sole aim of keeping Soviet control of the Sea of Japan- to destroy the other guy's power-projection fleet rather than building your own.
I assume you mean the OTL KM/IJN, not an ATL one. Otherwise, the easy option is to have the KM keep to Versailles limits, in which case the OTL Red Fleet is a match for it!