There is an app Halal Navi that has Halal restaurants listed. someone used it during their stay in Japan also, I found a YouTube channel "Halal food in Japan," which has many halal restaurant reviews.

Yes, we at FSJ (Four Seasons Japan) have developed our own application specifically for Muslim travelers to easily find halal food options. The app includes features such as locating the nearest mosques or checking if a restaurant has a prayer room. Additionally, our application provides detailed information about the restaurants, including when they were established, who will serve your food, and the most popular menu items. This is because we want Muslim travelers to enjoy their trip to Japan stress-free.