The Soviets might wait until the Americans attack, and when most of Japanese forces are engaged against US landings, Soviets might attack Hokkaido. Apart of sea landing, they might also attempt an airborne attack - a parachute brigade lands near a port, even a small one; a relatively y small force lands from sea in the same area; together they might be able take control of the port, which would allow the Soviets to form a solid beachhead and bring reinforcements even without big fleet of landing ships - a regular merchant ships or even fishing boats might be anough for a short voyage from Sakhalin and they can be unloaded in a port. Soviets also have total air superiority, with Red Air Force operating from Sakhalin. Hokkaido is well in range of Shtormoviks, providing good air support for troops on land. OTOH, assuming the Soviet attack starts in October/November 1945, days are short, which limits the effectivness of Soviet planes.
Of course Soviets would needed a lot of preparation for that and I'm not sure if they had time to do so. Persoally, however, I believe that had the decided to try, they had a decent chance of success. At least as far as Hokkaido goes.