Replacing stuff with other stuff

So, just for fun, I'm writing one of those custom PK3s that randomly replaces items and monsters with harder ones. I've got a couple of questions:

1. How do I replace a single item placed on a map with two items? I've seen this done on a mod before (can't remember the name of it, but it was on Doomworld).
2. How do I keep an item from spawning under certain conditions (for example; the ceiling is too low, the player doesn't have the right weapons yet, and so on)?

While I'm not completely sure if this thread belongs really to The Workshop one or any other r667s, I'll try to answer your questions:

1. Just try to use an actor that has properly states in it that will spawn 2/ copies of the same actor with A_Jump / A_SpawnItem tricks via decorate (jump for the state, spawn for the desired actor)
2. It's a bit tricky to explain I think, but surely you must use external compiled acs + CallACS functions to get rid such stuff. Sorry to not give to you any clue, I'm a bit busy ATM and I just wanted to be of some help for the purpose.