If you drilled the original pin for a finger-sized split ring, it should be serviceable again, you just wouldn't have the "lock up" position that plastic disc and fixed pin provide. And even that shouldn't be too hard to fabricate/find (a piece of plastic that would do the same job)
I assume you have one working version to copy??
The cotter ring is exactly what I was thinking if Rigrite isn't able to provide a replacement. The tip of the pin which extends beyond the car is hollowed out though, so it may not be a permanent solution due to wear.