I'm not advanced enough to tell you how to do this, but to simulate a koopa shell, you'd need an enemy that leaves behind a powerup, then that power up gives your player a projectile with one ammo, which shows the projectile as a projectile source when the shoot button is held, but doesn't create the actual projectile until the button is released.
Possible relevant scripts:
Monster drops: https://www.nesmakers.com/index.php...r-drops-are-health-when-you-have-1-left.6197/
Change character appearance: https://www.nesmakers.com/index.php...s-appearance-weapons-4-5-6-intermediate.7157/
Projectile shooting can be modified from the default input scripts.
Hopefully that gives you some ideas.