I'm not a big fan of the Permatex idea for footstraps screw, but here, since we're in fact talking about a "real thread" I would say it has a good chance of working. The main issue, as indicated by jc4 and carvesalot, will be how well it bonds to the fin box material. But after all, the fin box as a whole is glued in the board with epoxy and there is also a good irregular surface inside the hole (ie the old thread) that will increase the chances of a good bond.
I don't think a larger guage screw is a good idea. There is not enough material around the whole to support a bigger screw, I reckon.