Is this what you're talking about?
Nozzles
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Weathermatic brass nozzles
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Yes you can just swap out the guts. It's possible the sprinkler with the missing nozzle is damaged.
Or you could try just replacing the nozzle.
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I never ever ever see Weathermatic sprinklers but I believe the nozzles will also fit Hunter Pro Sprays and the Rainbird 1800 series.