General Antonyms:
* Law-abiding citizen
* Upstanding individual
* Model citizen
* Good Samaritan
* Honest person
* Respectable person
More Specific Antonyms:
* Gentleman/Lady (if referring to a male/female hoodlum)
* Scholar (if referring to a hoodlum's lack of education)
* Benefactor (if referring to a hoodlum's criminal acts)
The best antonym will depend on the specific context of how "hoodlum" is being used.