"Would you know how to find some reference on the speeds? Such as
metronome settings? I don't have a good feel how fast to play a Largo,
Lento, Andante, Allegro, Allegretto, Allegro Vivace, Allegro con brio,
Allegro brillante, etc."
That's a great question. I think a lot of people are not sure how fast should Allegro be. If you don't know what do Moderato or Allegretto mean then your performance might be a little out of character. Of course, it's always possible to listen to recordings and watch videos but then there is so much variety in tempos.
Also the acoustics of the building where the organ is located need to be taken into account as well as the style of music - while in a cathedral acoustics it would be OK to play some (but not all) virtuoso symphonic or modern music very fast, in the same acoustics Bach's fugue would sound quite confusing.
While usually mechanical metronomes should have tempo indications alongside with beats per minute written in, I guess some digital metronomes or smartphone apps lack this. In this case a musician would need to consult an outside source for precise tempo meanings.
If you want to find out the most common tempo indications and how many beats per minute do they take, here is a good list. Keep in mind that in various countries and periods metronome markings were different. Hence the faster than usual (even virtuoso) performance of some Franck's organ works (by Belgian organist Joris Verdin), for example.
Today's sight-reading piece is Moderato (p. 2) by Niels Gade (1817-1890) who was the Danish composer, violinist, organist, conductor and teacher, basically the most important Danish musician of his day. Since this composition is not for the performance, as you sight-read, don't worry about the tempo indication - just play at a speed that is comfortable to you, generally very slow.
Usually people sight-read too fast, if you know what I mean...