I still want to know about Garth of Izars super chemical explosive.
I doubt he had any. He was simply crazy as a cuckoo. Nothing we saw
suggested anything "super". It's a mystery to the audience why Governor Cory would confess to believing that Garth has a superexplosive...
In theory, new torpedoes can be manufactured, but the matter/anti-matter warhead may be impossible.
It's difficult to see why a starship, especially a TNG era one, would have any difficulty manufacturing anything
. A dedicated factory might just do it a bit faster and more efficiently... Whereas a seriously damaged starship like Voyager
would struggle with the task, at least until her replicators got repaired.
But DS9 "Tribunal" features a plot where the Maquis are framed for the theft of (empty) photon torpedo warheads, suggesting that everybody believes these to be worth stealing. Of course, this happens before the Maquis acquire industrial replicators (they make a big effort to steal those in "For the Cause", rather suggesting they had none previously), so perhaps there is some difficulty to manufacturing a working torpedo warhead.