To spawn a pipe when all sprites on screen are killed, you might want to edit the custom sprite called "door generator" It explains how to change what map16 block in spawns, so you could just change the spawning of the top part of a door to the left part of a pipe, then the bottom part to another section of pipe, then make another copy of the sprite for the right sides (Unless you can figure out how to do it all in one.)
Two more sprites for horizontal pipes.
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