But rather than a Japanese invasion of Siberia, how would the USSR fare if it was the aggressor, choosing to attack Manchuria in 1939? I'm sure that Japan would choose to defend Manchuria/Manchukuo as well as they could, considering the area's industrial importance, and proximity to the Home Islands. Not to mention the fact that there are all those divisions in China that could be called up, so I'm pretty sure that the IJA would put up much more of an effort.
What is the goal of the invasion? There isn't anything there important enough for Stalin to rish a second war while fighting the Finns.
As long as logistics work out (which, based on Finland, won't be overly long) the Red Army stacks up Japanese battalions like cordwood and advances at will. Once they hit the limits of their supply lines the Red Army combined arms formations would dig in while continuing to slaughter IJA light infantry until the Soviets run out of ammo or the Kwantung Army runs out of men.
That is the big problem for the IJA; it is a light infantry force, with no substantial artillery train or armored formations, lacking even decent ground transportation. In most ways it had, outside of infantry kit, not evolved from the force that fought the Russians at Port Arthur. While there were air units, even some light armor, the IJA had no combined arms doctrine worthy of the name (the air arm was supposed to sweep the sky of the enemy and bomb factories and other targets, with virtually no thought to close air support beyond occasional attacks against target of opprotunity). The IJA did okay when light infantry, with only minor support, was either all that was needed (China, many of the early actions against colonial forces) or where conditions were actively hostile to heavier forces (jungle fighting, at least until the heavier forces learned new ways to find, fix, and defeat IJA infantry units)
The Red Army (like the other WW II combatants) was a combined arm force, with a heavy reliance on artillery and armor, with organic air regiments. It had solid, if uninspired, doctrine for ground support and was well ahead of the Western Allies in use of armor and infantry as combined formations. Moreover, the Soviets had much of their "A" team in the Far East (what was left after Stalin had thinned the herd) up to and including Georgy Zhukov, who had defeated the Kwantung Army at Khalkhin Gol. (It is also interesting to note that the IJA used a TOTAL of around 190 light tanks during the entire year long campaign, while the Soviets used 500 during just the decisive battle, all of which greatly outclassed their Japanese counterparts.)
Light infantry vs. combined arms, artillery heavy formations. Easy to guess who comes out ahead.
God knows what would happen then.