The idea would be to prevent America from building a factory there. The attack would be with two infantry from Indochina, supported by fighters from there and Manchuria. (Japan would replaced the two infantry in Indochina during the non-combat phase.)
The chances of success, according to the combat simulator software, are about 70%, better than 50-50, although by no means certain. There is the possibility of America's recapturing it on its turn, but it then couldn't build the factory until at least the following turn (a country has to own a territory at both the beginning and end of its turn to do this). And even if Japan's attack didn't totally succeed, it would probably weaken the American defenses by "strafing" the troops there.
Is it worthwhile for Japan to try to disrupt the American factory building program? And even if this wasn't the issues (America doesn't like to build factories, or there is a no-factory "house rule,") is Sinkiang, which abuts on both India and Russia's Novosibirsk, strategic enough for an early attempt at its capture?