This patch will make disco shells appear if you add one of sprites DA, DB, DC, DD or DF with extra bit set to 1 (or 3, but that one is used by spritetool).
This version fixes a major bug in vertical levels.
I was idly poking around the other day and noticed that this patch doesn't set the "disable fireball/cape killing" tweaker bits for the spawned shell, making it less resilient than the vanilla version. Though, honestly, being able to stun a disco shell with a cape, carry it somewhere else and kick it to re-disco-ify it is kind of cool.
Fixing this tiny oversight is as simple as adding
right after the "INC $14C8,x" at the bottom of the patch.
It's about 8 years late, but thanks for this patch - even now this old thing is still a useful lesson on the weirdly hidden extra bit for vanilla sprites.