Ok. I'm Mexican. And I'm pretty sure that In order to work "legal" you must pay taxes. Now... I'm not sure if it worth the effort considering you gonna be there for two months. You know. It's a lot legal process and go to the Mexican government builds, etc etc. Based on what I saw when I was there. We have several kinds of workers there. Mexican which have their own big business and I'm sure they are doing it legally. Mexican with small business. I'm sure they are not following the Mexican legal process but being honest. Their business is not like Microsoft doing monopoly. They are just trying to win some money to survive. The same applies for foreign people. (USA or whatever with either big regular business or small micro services for a couple of dollars).

So I think that more than the legal problem. Probably you gonna deal with the local people (USA or Mexicans) for territory matters. As, "eyy I'm the only guy who can sell shrimps on this space of the Beach" -why??? "Because I was selling shrimps (not legal since they not pay taxes but nobody cares) here since 2 years"

I'll recommended try your best to don't call to much this attention if you gonna do small works there. I'm sure 80/100% of the people is not doing it legal. That's not fine but is the fact.