Thanks guys, went with the (larger, heavier, more powerful and more expensive, but in stock) GS battery. $3110 MXN with install and I'll be able to get back on the road tomorrow.
Can't say enough good things about Vesca Motors in Oaxaca. They're a new dealership and it's clear they're putting a lot of effort into creating an excellent customer experience, including the service manager letting me use his cell phone to call the US when my bank declined the foreign transaction...
They also did an amazing job of cleaning 8,000 miles of grunge off of it: