Sounds just fine to me, personally. It’s free maintenance work; may even be appreciated. If the original author returns someday and wants to maintain their own modules again, your bugfixed version would spare them some effort.
In general, before creating a derivative, you’ll want to check out the permissions settings on the project, and see whether the author made their views on derivatives being created clear elsewhere in the upload, like the description or in a readme file. Mind that migrations get uploaded flagged “Open - Free & open only if project also open” (i.e., tinker away, but don’t attempt to prohibit other people from tinkering with your derivative in turn), which may not necessarily be what the original author would have chosen had the option to place these flags existed at the time. When editing and redistributing a Migrate Wizard project, it’s always possible that somebody’ll be ticked off by it someday.
^- Permissions & licensing section on project page.
If you want to be thoroughly polite, then give clear and prominently-displayed credit in your upload description, explain why you did it, make it possible for the original author to contact you if they should want to, note that you’ve attempted to contact them beforehand (if you’ve attempted to contact them beforehand, which I take you have), and link back to the original upload. 's about all you can do on your end.
In case an original author is ever displeased to see what you’ve done, then be understanding and respectful about it rather than get defensive, and things should go just fine. “Your work was so great and I loved it so much that I didn’t want it to get rendered unplayable by patches in your absence” is a very flattering notion, all things considered. Plus, the upload entry can be handed over to them if they decide to come claim it someday, same as with the stuff currently linked to the Migrate Wizard.
This kind of thing usually goes over on a per-case basis that depends on the stances of the individual people involved, more than real fixed ‘protocol’, since nobody’s livelihood is on the line. Gestures of good faith, like asking around how to go about this kind of thing or how to reach a specific absent author, are very likely to be appreciated, though. Since you’re already inclined to go about it this way, I think you’re on a pretty safe road for avoiding unnecessary humandrama.
It’s like 99.9% just a “Try not to be a jerk and unnecessarily piss people off” + “Do have fun and build some neat stuff yay for videogame tinkering :-D” thing. Generally speaking, I think most authors just don’t want to feel like their efforts are going unappreciated, as in “Welp! Now that you’ve done the work and I have the product, who gives a damn about you anymore?!”. But most of the stuff hereabouts was made to be freely tinkered with, and this is a collaborative community where people build upon one anothers’ work on a daily basis, so don’t fret about it overmuch. If somebody bites you for trying to bugfix something so it remains usable, I’m pretty sure that’ll draw the more reproachful glances by far.
Authorship guidelines over here, Code of Conduct over here.