You need to fix this, I don't think you'll lose your mast, but the dolphin striker is an important structural element and this lets it flex.
You probably can't re-rivet in the existing holes, the holes are probably elongated in the beam.
Luckily this has been addressed by Hobie years ago, the replacement mast step has four rivets instead of two and they are in different positions than the two existing ones.
Check these pictures of replacing the mast step in the Beachcats Technical album.
Replacing Hobie 18 Mast Step

When you do yours, take pictures please and document any problems you run into to help the next H18 owner.
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Damon Linkous
Owner, TheBeachcats.com
Hobie 18 Magnum
Memphis, TN
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