I cannot find online manuscripts of Bach's fughettas, but manuscripts are available for the Well Tempered Clavier and the 15 Two Part Inventions. Manuscripts are the actual scores written out by Bach himself.
Those two manuscript sources do not have tempo indications. Certainly no metronome markings but also not even expression markings like "allegro."
At that time musicians had a general understanding about how something should be performed based on meter and the rhythmic value of notes, and also basic genres. As an example, a sarabande in 3/2 meter with lots of half notes was understood to played properly at a slow tempo. But a courante in the same 3/2 meter with lots of quarter notes and shorter values would be played at a brisk tempo. A lot of that was just understanding the conventions of those dance genres, but I think the performance of those dances probably informed the performance of other musical genres with similar meters and rhythm values.
This doesn't mean their were absolutely no tempo/expression markings. Plenty of baroque music would indicate 'vivace', 'gave', etc between musical sections or movements. But it's important to understand many pieces have no indication and there are assumed performance conventions.
I have an Alfred edition of Bach's Inventions which includes an index of metronome markings from about a dozen different previous editions. Some of those tempos differ by double!
I think it's perfectly acceptable for you to try a wide range of slow tempos. Given the title you want gravitas not sprightly (IMO.)