Should I replace this fixture box or get creative with the mounting?
Ceiling fans, when properly mounted, are put on specialized mounting/junction boxes that are especially strong, have a very secure mount for the fan and are hard screwed into a beam or stud.

What you have there looks like a pancake style fan box. See this example:



Every junction box has threaded holes for mounting fixtues such as your track lighting base. The studs in your picture appear to be the bolt holders for the fan.  If they are stripped, it is not safe to hang things from them and oversized bolts are a risky work-around.  

It may be that other screw holes in the bottom are properly threaded for standard fixture mounting. Check them using the bolts that came with the fixture.  If they are not (and there are usually several unthreaded holes in junction boxes), you should probably replace the box with a new one with intact mounting tabs/holes.