Re: boat restoration
Welcome to iboats!
Exterior grade plywood coated in epoxy will work just fine.
Are you sure that the transom is the only "need" ? You might want to check the stringers and deck too.
Read the completed projects section, use the search section and search for transom...... try glastron too.
I would guess you can spend hours of reading..... well days of reading but you are in the right place.
Once again welcome!!