That looks like a perfect example of how NOT to rebuild a transom.
Forum member Shine rebuilt an Aquasport 222 flat back a while ago. He had a very interesting approach to his transom rebuild. He used a sheet of melamine board as an outer form to lay up his outer skin and then laminated his core from the inside. If I was in your shoes, that's what I would do. His build thread on Boat Builder Central.com was converted to a tutorial here:
http://boatbuildercentral.com/howto/aquasport/index.php
The transom rebuild is on the first couple pages. If you go that way, you'll want to use a good mold release on your form. And personally, I wouldn't use his approach with anything less than epoxy resin. Polyester might work, but the superior secondary bond of epoxy would make me a lot more comfortable.
Dave