Wow, I would not start out on such an adventure without the bike being in absolutely top notch condition.
While it would be possible to take a vintage bike and completely rebuild it I suspect that you would be money ahead to get a newer bike. As for parts availability, especially outside the USA, I would certainly not go on a '70s vintage bike!
There are lots of smaller fourstroke bikes available, such as a Yamaha TTR-230 or TTR-125. You wouldn't want them, however. Many states have a lower limit on the engine size to be legal on the freeway and the bottom line is that you will want to be able to keep up with traffic and anything smaller than a 250 just won't.
You really don't need an off road bike as much as a bike that can handle some rough roads. A good dual sport bike would be what you want.
You should also consider that high octane gas can be hard to come by in Mexico so I would buy a bike that has a lower compression engine, 9.5 to 1 instead of the higher performance 11:1 that the competition bikes would have. If you are going a long way you will also want good street tires, not dirt tires. A big gas tank will come in handy.
I think that you want to go big, like a 650 or at least a 525. A bike that doesn't recommend changing the piston every 45 hours of use. An engine that allows you to ride at 50% throttle all day long so that you aren't pushing it too hard.
Unless you plan on flying home you need something that is very dependable and can survive for 10,000 miles. This is not a dirt bike......
Rod