So it turns out I had already reached out to MG Cycle. Gordon got back to me today and says that this is the correct part:
But we already have one that looks just like it, and it's too short. The description says it's 63.25"+3.25". ThisOldTractor.com doesn't have a parts catalog for this bike, but I managed to find this: https://www.parts-motoguzzi.com/moto-guzzi-motorcycles/1100-MOTO-GUZZI-MOTORCYCLES/CALIFORNIA/1998/California-EV-V11-EV-USA-/
According to that, the part number is 03093000. One domestic site shows it as 58.5" + 4" (and wants $90 for it). But a document I found on a French Moto Guzzi site indicates it's 1593 mm x 1440 mm (62.7" x 56.7", or 56.7" + 6"), which sounds a whole lot closer to what we need:
I'm praying we can find the original to compare it to. Unfortunately, the process is basically break-the-cable/order-new-part/wait-for-part/find-time-to-work-on-it/install-new-cable. Unfortunately, with typical Oregon winters and personal schedules, that second-last step takes a while. I'm afraid that someone threw away the old cable somewhere along the line.