you should be able to just download it via SideQuest - when you go to the 'buy' button it should open a browser window with itch that you can use to sign in and select download - from there SideQuest should ask you if you want to install, just hit yes.
if that path isn't working, you can download the .zip from itch, unzip it (you'll see a TOTHETOP.apk inside), then drag that .apk file into the SideQuest window and it should install.