As a long long time runner I have had 2 pelvic stress fractures over 20 years ago. They were in the ring of the pelvis and one I actually took only a week off from running and returned slow. The Dr said this was only possible because the pelvis is a double ring and found out after it had already healed. The other keep me from running for 3 months. A femolar stress fx is rare and you really would have had to over-do it. I was still able to cycle and I would do what the Dr says but most stress fxs are weight issues and cycling does not present this problem. Being completely inactive is not always the best because of blood flow and the healing that promotes. Do not listen to me but I wonder if you could simply spin very easy in on a trainer and no standing.
Pain is the body's indicator and just how bad is the pain. I ran through the discomfort of my first stress fxs after taking a week off running. I went to the the ortho doc who did not do any xrays or test other than hands on with movement. I could run with discomfort but the second one was pain.......I could barely walk the dog for a week and then it gradually got better. I can run with probably major discomfort but pain was impossible. Remember, God heals and the doctor sends the bill. If you can cycle/spin easy with no pain then personally that might be fine. Even a week of rest then go back spinning slow.
I also ask the doctor at the time if I took nsaids could they mask the pain so much that I would do damage. He said no. Taking something like prox/sodium helps and if you have no stomach issue that is the normal treatment with rest. THe dr had me taking them and then tapering them down as pain got better over time.
BTW, don't stop running when it heals but if you need distance for a marathon you can run shorter and add long bike rides. I have not run over 13 miles in the past year but could do a marathon without too much issue. You can simulate a 20 mile training run by running 12 miles then biking 50 miles right after the run.