Even given the above, the Japanese are largely doomed. The problem is that they are fighting in a huge expanse of what is essentially barren wilderness. Their army lacks the necessary means to support large scale operations logistically over the kinds of distances involved here.
Japan's Navy is essentially useless for a Sino-Soviet campaign. They could contribute some air units but that is about the extent of their use.
The Soviets could easily give some ground in exchange for causing the Japanese more casualties and making their operations more difficult.

If anything, the biggest effect is putting the stipulation of no American support to Russia or the Allied forces in Europe. This has a greater impact than the inclusion of Japan in operations against Russia does.
Of course, this does stretch the bounds of reality quite a bit to happen.