Welcome to Wayfarer sailing, a hobby executed by a bunch of people that loves the equivalent of having a shower with their clothes on while tearing up 100 pound notes.
The pivot hole is actually 16 mm wide (maximum measure) and the mast pin is minimal 6 mm. Its function is to limit the amount of mast movement so we all race under similar conditions. Racers must have the pin mounted. Other then that the pin is a useless piece of equipment that only gets in the way of a real good trim, though it does play a helpful role when one wants to lower his mast.
There are fancy and expensive stainless steel rods you could use, with holes drilled in the ends for shiny stainless key rings. I believe you can order them from the builders. An alternative is found in any DIY shop; a galvanised 6 mm steel bolt of sufficient length and a wing nut. To top it of, add a couple of large 6 mm washers. And yes, galvanised steel does rust but if you replace the bolt every year it takes fifteen years (and bolts) to come close to the price of an “official” SS-rod, unless you make one yourself of course.