My three cents, I have built three planes with the older Italian S/T's and have not been able to get anyone of them to run correctly inverted (lot's of dead stick's). I live at 5000' have an OS 90 FS inverted in it, run's absolutely beautifully. If you look at the opening you must have for the exhaust for the S/T inverted it is probably more than the head opening you need for the OS90. You can twist the exhaust on the OS so it requires very little opening.