If it's just in a couple of places, you might want to ask if it really needs to be addressed. Leaving in a couple of performance mistakes like that can sometimes make the performance sound more "organic" or give it more character than trying to find the perfect grid.
If it's such a bad performance where the fret buzz is pervasive, then you pretty much have a bad take that really should be replaced. I know you said that's not an option in this case, but it's probably what you're stuck with.
It's a longshot that's may not be worth the effort, but if you are all out of other options you might want to try and see if you can ID the buzz as a harmonic series using a frequency analyzer. If so, you might be able to attack it somewhat either with an EQ that has a harmonic filter function or with a series of narrow Q parametric EQ cuts attacking the culprit frequencies.
The problems/tricks with this this idea is that the buzz may just be too complicated to filter easily that way, and/or, even if you can filter it fairly well, you may have a hard time completly cutting out the "cancer" sound without removing too much of the "good tissue" sound. That is, you might have to shred the good sound of the guitar by trying to remove the buzz.
G.