It would be a foam frame wouldn't it?

Is the deck flexing lots in that area? Can you hear it going crunch when you push down over the spot?

Decks aren't that hard to replace on a Mosquito, if you had to tool up for the job and buy bulk quantities of consumerables ( much cheaper that way), enough to keep the boat on the water for years, I would be surprised if it cost more then $500. Thats including buying a router and sander.

If you are going to race a plywood Mosquito you will need to aquire the skills sooner or later. If not it will be cheaper to buy a glass boat now.

Darryn