My sister's first husband would not hesitate to take on that repair. He made Fiberglas tools to build airplane parts and always seemed to have some genius idea to repair stuff.
I see what you're saying about screwing in some pins and glassing it with epoxy. It's going to be an ugly repair but that rudder has most defs seen better days. I'm thinking you could make a mold, or imprint, of the other rudder to get the shape. Maybe make a half-cast mold of the right side of the rudder with plaster. Drill holes for some long pins in the broken face. Place the rudder into the mold and fill the mold with Bondo. When the Bondo has hardened, sand the rough left side to match the molded right side. What pins to use? Three-inch stainless, maybe?? Then I would fiberglass the patch using epoxy resin instead of polyester resin. If you have a West Marine nearby, I'd take the broken rudder to them and ask which of their array of resins, fibers, and putties to use. But before that I'd spend at least 10 hours watching YT videos from the
Fish Bump TV channel on using fiberglass, resins, and peel ply.