Are there any mutagens that can undo the mutations they cause?
It is possible but extremely unlikely.

When a base undergoes tautomeric shift the DNA does not contain a mutation yet, just an unmatched pair. The mutation will only becomes inscribed into the DNA permanently after the DNA is replicated or wrongly repaired.

In order to reverse the mutation you would need to provoke a chemical change to that specific base using the same or another mutagen and as most of them are unspecific the likelihood of obtaining both the correct mutation to reverse the effect AND at that specific location is extremely low. Just for the location itself (i.e. that base pair), in human you have ~0.000000033%  chances (1/3 billions) to mutate that base. This is assuming equal probabilities across the genome and a single modification event which is of course not true but shouldn't modify much the conclusion. I am not even speaking about provoking the chemical modification that will reverse it which will even further reduce that probability.

Such induced or natural mutations will actually only rarely appear in the genome permanently as the cell has multiple DNA repair mechanisms (see this wiki). So in this case the modified base does not need to be reverse as the cell will make sure to repair it directly but using mechanisms not based on mutagenesis.