Its far from given everyone will jump to save Poland. Nobody east of Saarbrucken liked the Second Republic, and if Stalin plays his cards right, he might get away with inforcing at least the Curzon line.
Actually, Poland had very friendly relationship with both Hungary and Romania. In the case of Romania both countries had military alliance in case of war with Soviet Union. In case of war against the USSR of either of these countries, the other side was obliged to start combat against the USSR with minimum 14 infantry divisions and 2 cavalry divisions within 18 - 28 days.
Polish war with Soviet Union would be different from the war in 1939 with German
*The terrain in the East is ideal for defense-swamps, forests and very little transportation infrastructure
*Poland prepared for defense against Soviet Union since 1920s, the plans for war with Germany were barely prepared when the war started
*there was an impressive line of fortifications in the East prepared for defensive war, nothing like that in the West
*the Soviets had poorly trained soldiers, in 1939 they had problems fighting with Polish civilian militias
In the long turn the sheer number and potential of Soviet forces would outweight Polish strenghts, but the campaign would be difficult and longer.
Plus I think that most conservative elements of European politics would get the idea of "Bolshevik tide trying to conquer Europe". So the conflict wouldn't be so localised. Since both Romania and Poland-both cornerstones of French politics would be involved, France would likely get involved as well. Support from Italy, Hungary is certain.