Hi Drew. The kickstand plate is not really providing any structural support to the frame — many Rivendell models didn’t even have them — so the bike is completely safe to ride.
It’s actually fairly common for these plates to crack, peel away from, or shear off the chainstays on Rivendells. It happened on one of mine as well. Will at Riv told me it was nothing to worry about structurally. Riv later reinforced the design of these plates, which made them less prone to cracking or separating.
That said, it is unsightly, and you don’t want corrosion to deepen over time. I would either:
1. Treat the rust with a rust converter to stabilize it and create a protective surface that could then be painted, or
2. Carefully cut the plate off as close to the stays as possible using a Dremel or similar cutting tool and then treat the cut surface with paint, clear nail polish, or some clear coat. Hire the frame builder to do this if you're not comfortable.
Both of these options are much easier and less expensive than fabricating and affixing a new plate.
If you normally run a kickstand, I’d recommend using one of the
chainstay “sandwich” style mounting sets that Riv sells instead. If you leave the existing plate in place after treating it with rust converter, the mounting bolt would simply pass through the hole, bypassing the plate itself as the kickstand support.