Replacing top deck
To attach the top deck you could use 3M 5200 adhesive caulk.
You can also use epoxy resin like West Systems with a milled fiber mixed in for thickening.
It is also possible to get the green adhesive used by seadoo, but it requires a large gun to use, and their minimum order is was $400 the last time I got it, for just the 2-part glue tubes. The gun is crazy, too.
Kurt