This game has separate 'Fire' and 'Bomb' (missile) buttons. I learned that you can map both to the same button on the MiSTer by first defining all buttons except the 'Bomb' button, and then set up alternate controls and assigning 'Bomb' to the same controller button as 'Fire'.
Screenshots marked with 🍒 are created by syltefar and are considered public domain, free to use for anything. If you want to, you can note where you found it and link to this page.