I believe that most, if not all, of the studs in the case are DIN 835 standard. This means that the "tap" end is 2x the diameter of the stud. For example, an M8 stud has a tap end of 16mm. This is the end that is screwed in to the case. The other end is the nominal length of the stud.
You can identify the stud from the parts catalog...
Pos #4 on the parts diagram is an M8 x 35mm stud.
So, the height of this stud in this position is 35mm.
Some words of caution...First, there are mistakes in the parts catalog, so be careful. Second, most of the studs you can buy
are not DIN 835 and will not match exactly. Meaning, that the nominal length of a stud might be 35mm, but you may only be able to find it in 36mm. Third, a lot of these motors have been apart a few times by now and may not have the correct studs in them to use as a reference. I crosschecked all of mine against the parts catalog.
I measured and took pics of every stud in my case before I removed them. If you post a pic of the missing stud, and I might be able to help you.