I think the proper way would be to a point-in-restore of the filegroup. That would bring back the original files. Then again, given that they are infected, maybe you don't want them back.
Now you still have a corrupt database in some meaning, although SQL Server does really notice. But if you would modify these files with an UPDATE, and you would later restore a backup from before the AV incident and apply logs, you might get a mess, because this will restore the files that were never updated.
Then again, you could argue that this is largely academic.