Mogas and Avgas "Octane" numbers do not equate exactly, the test methods are different. However, this is moot, the B-17 was not and should not run Mogas. 100LL (blue) and 100/130 (green) are both 100 octane where it matters. 155/145 (purple) is long ago not available. If there were a concern about inadequate octane rating of the Avgas for an engine which was built to use 115/145, reducing a combination of power, duration at power, and temperatures could restore detonation margin. If in doubt, run the engine at a faster RPM, and lower MP, rather than the other way around (don't lug the engine) and it can tolerate lower octane, particularly for a brief time. Certainly, if I were flying a MP limited engine(s) and needed the power to prevent crashing, I'd push the throttles further forward. It might detonate, it might not - to save the plane, it's worth the risk, though you might be replacing some pistons afterward.
100LL Avgas should be the same worldwide. Mogas not so, different by different national standards, and often has stuff other than "gasoline" blended in.
I have conducted detonation testing on a Continental 470, 520, and Pezetel engine, and succeeded in making the engines detonate, and measuring the event. I did it without damaging any engine doing the testing. That said, I only detected the detonation by my measurements, and watching engine temps very closely, I could not tell that detonation had begun by "feel" or engine noise, nor smoothness/performance. Once I had detonation, I immediately reduced power to stop the detonation. Interestingly, the detonation continued for seconds after the power was greatly reduced. I would never fuss the maximums and risk detonation in service, other than to prevent an immanent crash. From this testing, I found the manufacturer's maximum operating limitations to provide conservative detonation margins. Cesar Gonzales, who did a lot of detonation testing for Cessna told me that Cessna melted pistons and trashed engines occasionally during their testing.