"Les habitués d'ici ont la panse bien remplie!" or

"Les fèves ici sont pleines d'énergie!" or

"Des fèves, en veux-tu en voila!"

just a few suggestions...

This is obviously a "jeu de mots". The expression "full of beans" means "to be lively", "to be in high spirit", or in French "être gaillard", "être plein d'entrain", "se porter à merveille" (this, from my Oxford dictionary and from my Harrap English-French dictionary.) But this is also a Mexican restaurant, therefore they are saying that when punters are there, they get filled with beans, the staple of this country.

As for "punters" it can mean different things. You may want to know where that restaurant is situated. If it is near a football field, "punter" has more chance to refer to a kicker, and will be translated with "botteur". But if there is a canal nearby, with some of those flat bottom boats which are propelled with a pole, the punter is more likely to refer to the person who "punts" one of these boats (not sure of the name in French, though...) If the restaurant is near a race track, the people more likely to get full of beans there are the "parieurs". You will have to decide yourself what the word "punter" refers to. Failing that, you can always use something safe such as "clients" or "dîneurs" or "habitués".

This kind of double meaning is always very difficult to translate, especially when it involves an expression with imagery such as this one.

You won't find an equivalent expression, so you will have to invent another one that has somewhat of a similar feeling. I proposed a few above, or you may be able to think of something better along the lines of feeling full of energy as well as feeling full and consuming good typically Mexican food.

This is not easy. But it is fun to be creative in translation. Good luck!