the scarf joints in my boat were done at home using a hand plane and cleaned up with a sanding block, using a technique found on the internet, Google - Brian dixon's Easy Plywood scarf bevel cutting technique.
use west resin to glue, i sat mine on the floor of the shed with polystyrene under it to stop it getting cold from the concrete.
my hulls use a hull doubler that covers the main scarf joint at the main beam, as used in the origina plans and the Ken Lott plans.