You can only attempt getting Zapdos once but if you save your game before trying to get it and let's say you fail then just reset the game and reopen your file you will be able to fight Zapdos again and retry to capture it.
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Yes if you saved before attempting to capture him if you didn't save your out of luck.
You can find Zapdos in Power Plant only once so if you let Zapdos fly away or if you defeat it you can never find it again in LeafGreen.
That item doesn't exist in leafgreen.
Piplup does not exist in Pokemon LeafGreen or FireRed.
Psyduck is in firered not leafgreen you can find it using a good or super rod in the safari zone.