1. You can buy Mexican insurance before you enter Mexico at the Mexican border. There are tons of insurance shops near the border. Go to a couple and ask for the rates. If you are going to be in mexico for a long time, it may be cheaper to buy a policy for one year. Runs about $120 for one year liability only.
2. No carnet de passage is required for Mexico. Do bring your Drivers License, Passport, title for the motorcycle, and a credit card (Mastercard, Visa, maybe Amex) in your name. If you don't have a title, they often accept a vehicle Registration paper instead. Fees for a TVIP (Temporary Vehicle Import Permit) is $29 for 90 days. You will also need to get a tourist card if you plan on staying in Mexico for more than 72 hours. A tourist card is $20. You get both right after you cross the border into Mexico.
--Terry (just left Mexico yesterday, now in Belize, on the wonderful Caye Caulker)