If I'm not mistaken... Issun-Boshi was swallowed by an oni as well, and he simply cut his way out of the oni's stomach. When the oni fled in terror, he left behind the Miracle Mallet.
If you has read the legend about inches, then we would know what will happen next. Sukuna would attack Kasen with her needles from inside. (Very hurts)
Now imagine Sukuna rampage through Kasen's belly like what Durmite does toBowser in Bowser's inside story. The snacks that Kasen ate = The giga carrots that Bowser ate Sukuna = Durmite，Kasen = Bowser(Oni from = Giant Bowser)
