This is my effort (with thanks to Kinson): "Inside each man can be found everything he needs to deal with whatever life may throw at him. Whenever he encounters adversity, he smiles. Whenever he encounters folly, he laughs." My style is a little literary, but I think it suits the context. It's also old-fashioned to use male pronouns only ("he", "him") but I think it sounds better if you just choose male or female.

This meaning is not clear to me. Does the big belly represent a knowledgeable man? Does "open his mouth" mean to enlighten people and spread the truth? Are the "men who should be laughed at" fools or crooks or con men? Does "laugh at them" means to reveal their folly? Does it mean that a wise man can easily dispel (or reveal) the folly of others?