1. Beat those guys up, and be really harsh with them. And I mean really harsh to put fear into their minds, that lasts for a lifetime, but let them live.
2. Tell the smith to pack his valuables and granddaughter and leave by fishing boat.
3. Follow the beat up goons to their HQ/boss/hideout/whatever, because they WILL report to someone and follow that guy.
4. If you find the boss, good. In case the trace gets lost, either beat up the goons, you did find, (yet again give them the rough treatment) and continue to go around the city and beat up more and/or wait and make your location openly known. Something is 'bout to happen either way. (Just to make sure that thing isn't the granddaughter being taken hostage, find her first and get her out of the city with her grandfather, in case they didn't leave already after step 2.)
5. MC saved a kingdom, so he should secretly contact his friends in higher positions by a secure channel (adventure guild or something), so that a friendly face will arrive soon.
6. If finally the boss turns up, all is good.
6a. If the local authorities show up and try to pin everything on MC, all the better! The real boss is either the local lord or the town guard captain. Go to the lord and confront him. Don't mention, who you are first. Just see where it goes. Resitst every attempt to be arrested. Rought 'em up a little and go drinking a coffee at a cafe or something.
In the end of the day MC would have beaten up half of all goons in the city, disrupting the operations of that organisation majorly and might have uncovered major corruption in that city. Everything with the awesome power of violence! There is no need to be delicate with a crime organisation, that is acting that vile and is into people trafficking. There is a point about being more cautious about the government/nobles, but not anymore if they are obviously involved, as they will lose their titles anyway soon and MC is backed by higher authorities.